idnits 2.17.1 draft-ietf-emailcore-rfc5321bis-08.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The abstract seems to contain references ([5321bis]), which it shouldn't. Please replace those with straight textual mentions of the documents in question. == There are 2 instances of lines with non-RFC2606-compliant FQDNs in the document. -- The draft header indicates that this document obsoletes RFC7504, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document obsoletes RFC7505, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document obsoletes RFC1846, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Line 2077 has weird spacing: '...ivalent and...' -- The document date (31 December 2021) is 847 days in the past. Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '5321bis' on line 3107 -- Looks like a reference, but probably isn't: 'JcK 20210904' on line 2637 -- Possible downref: Non-RFC (?) normative reference: ref. '2' ** Obsolete normative reference: RFC 821 (ref. '3') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 822 (ref. '13') (Obsoleted by RFC 2822) -- Obsolete informational reference (is this intentional?): RFC 974 (ref. '16') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 1869 (ref. '22') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 2821 (ref. '30') (Obsoleted by RFC 5321) -- Obsolete informational reference (is this intentional?): RFC 3501 (ref. '36') (Obsoleted by RFC 9051) -- Duplicate reference: RFC5321, mentioned in '51', was also mentioned in '47'. Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMAILCORE J. Klensin 3 Internet-Draft 31 December 2021 4 Obsoletes: 5321, 1846, 7504, 7505 (if approved) 5 Intended status: Standards Track 6 Expires: 4 July 2022 8 Simple Mail Transfer Protocol 9 draft-ietf-emailcore-rfc5321bis-08 11 Abstract 13 This document is a specification of the basic protocol for Internet 14 electronic mail transport. It (including text carried forward from 15 RFC 5321) consolidates, updates, and clarifies several previous 16 documents, making all or parts of most of them obsolete. It covers 17 the SMTP extension mechanisms and best practices for the contemporary 18 Internet, but does not provide details about particular extensions. 19 The document also provides information about use of SMTP for other 20 than strict mail transport and delivery. This document replaces RFC 21 5321, the earlier version with the same title. 23 // JcK 20211029 Note in Draft: Adjusted in version -06. Decided the 24 // details belong in either the Introduction or the A/S, not the 25 // Abstract. And it makes the Abstract a tad shorter, which is good. 27 Notes on Reading This Working Draft 29 This working draft is extensively annotated with information about 30 changes made over the decade since RFC 5321 appeared, especially when 31 those changes might be controversial or should get careful review. 32 Anything marked in CREF comments with "[5321bis]" is current. In 33 general, unless those are marked with "[[Note in Draft", in the 34 contents of an "Editor's note", or are in the "Errata Summary" 35 appendix (Appendix H.1, they are just notes on changes that have 36 already been made and where those changes originated. As one can 37 tell from the dates (when they are given), this document has been 38 periodically updated over a very long period of time. 40 As people review or try to use this document, it may be worth paying 41 special attention to the historical discussion in Section 1.2. 43 This evolving draft should be discussed on the emailcore@ietf.org 44 list. 46 Status of This Memo 48 This Internet-Draft is submitted in full conformance with the 49 provisions of BCP 78 and BCP 79. 51 Internet-Drafts are working documents of the Internet Engineering 52 Task Force (IETF). Note that other groups may also distribute 53 working documents as Internet-Drafts. The list of current Internet- 54 Drafts is at https://datatracker.ietf.org/drafts/current/. 56 Internet-Drafts are draft documents valid for a maximum of six months 57 and may be updated, replaced, or obsoleted by other documents at any 58 time. It is inappropriate to use Internet-Drafts as reference 59 material or to cite them other than as "work in progress." 61 This Internet-Draft will expire on 4 July 2022. 63 Copyright Notice 65 Copyright (c) 2021 IETF Trust and the persons identified as the 66 document authors. All rights reserved. 68 This document is subject to BCP 78 and the IETF Trust's Legal 69 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 70 license-info) in effect on the date of publication of this document. 71 Please review these documents carefully, as they describe your rights 72 and restrictions with respect to this document. Code Components 73 extracted from this document must include Revised BSD License text as 74 described in Section 4.e of the Trust Legal Provisions and are 75 provided without warranty as described in the Revised BSD License. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 80 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . 7 81 1.2. History and Context for This Document . . . . . . . . . . 7 82 1.3. Document Conventions . . . . . . . . . . . . . . . . . . 9 83 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . 9 84 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 9 85 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 12 86 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . 12 87 2.2.2. Definition and Registration of Extensions . . . . . . 13 88 2.2.3. Special Issues with Extensions . . . . . . . . . . . 13 89 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . 14 90 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . 14 91 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 14 92 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . 15 93 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . 15 94 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . 15 95 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . 16 96 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . 17 97 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 17 98 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 17 99 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . 18 100 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 18 101 2.4. General Syntax Principles and Transaction Model . . . . . 19 102 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . 20 103 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 20 104 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 21 105 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 22 106 3.4. Address Modification and Expansion . . . . . . . . . . . 24 107 3.4.1. Forwarding for Address Correction or Updating . . . . 24 108 3.4.2. Aliases and Mailing Lists . . . . . . . . . . . . . . 25 109 3.4.2.1. Simple Aliases . . . . . . . . . . . . . . . . . 26 110 3.4.2.2. Mailing Lists . . . . . . . . . . . . . . . . . . 26 111 3.5. Commands for Debugging Addresses . . . . . . . . . . . . 27 112 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 27 113 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . 29 114 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . 30 115 3.5.4. Semantics and Applications of EXPN . . . . . . . . . 30 116 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 30 117 3.6.1. Mail eXchange Records and Relaying . . . . . . . . . 31 118 3.6.2. Message Submission Servers as Relays . . . . . . . . 31 119 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 32 120 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 32 121 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 33 122 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 33 123 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 33 124 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 34 125 3.8. Terminating Sessions and Connections . . . . . . . . . . 34 126 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 35 127 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 35 128 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . 35 129 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) . . . . . . 36 130 4.1.1.2. MAIL (MAIL) . . . . . . . . . . . . . . . . . . . 37 131 4.1.1.3. RECIPIENT (RCPT) . . . . . . . . . . . . . . . . 38 132 4.1.1.4. DATA (DATA) . . . . . . . . . . . . . . . . . . . 40 133 4.1.1.5. RESET (RSET) . . . . . . . . . . . . . . . . . . 41 134 4.1.1.6. VERIFY (VRFY) . . . . . . . . . . . . . . . . . . 42 135 4.1.1.7. EXPAND (EXPN) . . . . . . . . . . . . . . . . . . 42 136 4.1.1.8. HELP (HELP) . . . . . . . . . . . . . . . . . . . 42 137 4.1.1.9. NOOP (NOOP) . . . . . . . . . . . . . . . . . . . 43 138 4.1.1.10. QUIT (QUIT) . . . . . . . . . . . . . . . . . . . 43 139 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 44 140 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . 46 141 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 48 143 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 50 144 4.2.1. Reply Code Severities and Theory . . . . . . . . . . 51 145 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . 54 146 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . 55 147 4.2.4. Some specific code situations and relationships . . . 57 148 4.3. Sequencing of Commands and Replies . . . . . . . . . . . 59 149 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 59 150 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 60 151 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 62 152 4.4.1. Received Header Field . . . . . . . . . . . . . . . . 62 153 4.5. Additional Implementation Issues . . . . . . . . . . . . 66 154 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . 66 155 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . 67 156 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . 68 157 4.5.3.1. Size Limits and Minimums . . . . . . . . . . . . 68 158 4.5.3.1.1. Local-part . . . . . . . . . . . . . . . . . 68 159 4.5.3.1.2. Domain . . . . . . . . . . . . . . . . . . . 68 160 4.5.3.1.3. Path . . . . . . . . . . . . . . . . . . . . 68 161 4.5.3.1.4. Command Line . . . . . . . . . . . . . . . . 68 162 4.5.3.1.5. Reply Line . . . . . . . . . . . . . . . . . 69 163 4.5.3.1.6. Text Line . . . . . . . . . . . . . . . . . . 69 164 4.5.3.1.7. Message Content . . . . . . . . . . . . . . . 69 165 4.5.3.1.8. Recipient Buffer . . . . . . . . . . . . . . 69 166 4.5.3.1.9. Treatment When Limits Exceeded . . . . . . . 69 167 4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . 70 168 4.5.3.2. Timeouts . . . . . . . . . . . . . . . . . . . . 71 169 4.5.3.2.1. Initial 220 Message: 5 Minutes . . . . . . . 71 170 4.5.3.2.2. MAIL Command: 5 Minutes . . . . . . . . . . . 71 171 4.5.3.2.3. RCPT Command: 5 Minutes . . . . . . . . . . . 71 172 4.5.3.2.4. DATA Initiation: 2 Minutes . . . . . . . . . 71 173 4.5.3.2.5. Data Block: 3 Minutes . . . . . . . . . . . . 71 174 4.5.3.2.6. DATA Termination: 10 Minutes. . . . . . . . . 71 175 4.5.3.2.7. Server Timeout: 5 Minutes. . . . . . . . . . 72 176 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . 72 177 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 74 178 5. Address Resolution and Mail Handling . . . . . . . . . . . . 74 179 5.1. Locating the Target Host . . . . . . . . . . . . . . . . 75 180 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 77 181 6. Problem Detection and Handling . . . . . . . . . . . . . . . 77 182 6.1. Reliable Delivery and Replies by Email . . . . . . . . . 77 183 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . 78 184 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . 79 185 6.4. Compensating for Irregularities . . . . . . . . . . . . . 79 186 7. Security Considerations . . . . . . . . . . . . . . . . . . . 81 187 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 81 188 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 82 189 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . 83 190 7.4. Mail Rerouting Based on the 251 and 551 Response Codes . 83 191 7.5. Information Disclosure in Announcements . . . . . . . . . 84 192 7.6. Information Disclosure in Trace Fields . . . . . . . . . 84 193 7.7. Information Disclosure in Message Forwarding . . . . . . 84 194 7.8. Local Operational Requirements and Resistance to 195 Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 84 196 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 85 197 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 85 198 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 86 199 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 87 200 10.1. Normative References . . . . . . . . . . . . . . . . . . 87 201 10.2. Informative References . . . . . . . . . . . . . . . . . 88 202 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 92 203 Appendix B. Generating SMTP Commands from RFC 822 Header 204 Fields . . . . . . . . . . . . . . . . . . . . . . . . . 92 205 Appendix C. Placeholder (formerly Source Routes) . . . . . . . . 94 206 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 94 207 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 94 208 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 94 209 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 95 210 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 97 211 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 98 212 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 98 213 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 99 214 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 99 215 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 100 216 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 100 217 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 100 218 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 101 219 Appendix G. Other Outstanding Issues . . . . . . . . . . . . . . 101 220 G.1. IP Address literals . . . . . . . . . . . . . . . . . . . 102 221 G.2. Repeated Use of EHLO (closed) . . . . . . . . . . . . . . 102 222 G.3. Meaning of "MTA" and Related Terminology . . . . . . . . 103 223 G.4. Originator, or Originating System, Authentication . . . . 103 224 G.5. Remove or deprecate the work-around from code 552 to 452 225 (closed) . . . . . . . . . . . . . . . . . . . . . . . . 103 226 G.6. Clarify where the protocol stands with respect to 227 submission and TLS issues . . . . . . . . . . . . . . . 103 228 G.7. Probably-substantive Discussion Topics Identified in Other 229 Ways . . . . . . . . . . . . . . . . . . . . . . . . . . 104 230 G.7.1. Issues with 521, 554, and 556 codes (closed) . . . . 104 231 G.7.2. SMTP Model, terminology, and relationship to RFC 232 5598 . . . . . . . . . . . . . . . . . . . . . . . . 104 233 G.7.3. Resolvable FQDNs and private domain names . . . . . . 104 234 G.7.4. Possible clarification about mail transactions and 235 transaction state . . . . . . . . . . . . . . . . . . 104 236 G.7.5. Issues with mailing lists, aliases, and forwarding . 105 237 G.7.6. Requirements for domain name and/or IP address in 238 EHLO . . . . . . . . . . . . . . . . . . . . . . . . 105 240 G.7.7. Does the 'first digit only' and/or non-listed reply 241 code text need clarification? (closed) . . . . . . . 105 242 G.7.8. Size limits (closed) . . . . . . . . . . . . . . . . 105 243 G.7.9. Discussion of 'blind' copies and RCPT . . . . . . . . 105 244 G.7.10. Further clarifications needed to source routes? . . . 106 245 G.7.11. Should 1yz Be Revisited? (closed) . . . . . . . . . . 106 246 G.7.12. Review Timeout Specifications . . . . . . . . . . . . 106 247 G.7.13. Possible SEND, SAML, SOML Loose End (closed) . . . . 106 248 G.7.14. Abstract Update (closed) . . . . . . . . . . . . . . 106 249 G.7.15. Informative References to MIME and/or Message 250 Submission (closed) . . . . . . . . . . . . . . . . . 106 251 G.7.16. Mail Transaction Discussion . . . . . . . . . . . . . 107 252 G.7.17. Hop by hop Authentication and/or Encryption 253 (closed) . . . . . . . . . . . . . . . . . . . . . . 107 254 G.7.18. More Text About 554 Given 521, etc. . . . . . . . . . 107 255 G.7.19. Minimum Lengths and Quantities . . . . . . . . . . . 107 256 G.8. Enhanced Reply Codes and DSNs . . . . . . . . . . . . . . 107 257 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 108 258 G.10. Internationalization . . . . . . . . . . . . . . . . . . 108 259 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 109 260 G.12. Extension Keywords Starting in 'X-' (closed) . . . . . . 109 261 G.13. Deprecating HELO (closed) . . . . . . . . . . . . . . . . 109 262 G.14. The FOR Clause in Trace Fields: Semantics, Security 263 Considerations, and Other Issues . . . . . . . . . . . . 110 264 G.15. Resistance to Attacks and Operational Necessity 265 (closed) . . . . . . . . . . . . . . . . . . . . . . . . 110 266 G.16. Mandatory 8BITMIME . . . . . . . . . . . . . . . . . . . 111 267 Appendix H. RFC 5321 Errata Summary and Tentative Change Log . . 111 268 H.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 111 269 H.2. Changes from RFC 5321 (published October 2008) to the 270 initial (-00) version of this draft . . . . . . . . . . . 113 271 H.3. Changes Among Versions of Rfc5321bis . . . . . . . . . . 114 272 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 273 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 114 274 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to 275 -02 . . . . . . . . . . . . . . . . . . . . . . . . . 114 276 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 277 to -03 . . . . . . . . . . . . . . . . . . . . . . . 115 278 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) 279 to draft-ietf-emailcore-rfc5321bis-00 . . . . . . . . 115 280 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 281 (2020-10-06) to -01 . . . . . . . . . . . . . . . . . 115 282 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 283 (2020-12-25) to -02 . . . . . . . . . . . . . . . . . 116 284 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 285 (2021-02-21) to -03 . . . . . . . . . . . . . . . . . 116 286 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 287 (2021-07-10) to -04 . . . . . . . . . . . . . . . . . 118 289 H.3.9. Changes from draft-ietf-emailcore-rfc5321bis-04 290 (2021-10-03) to -05 . . . . . . . . . . . . . . . . . 118 291 H.3.10. Changes from draft-ietf-emailcore-rfc5321bis-05 292 (2021-10-24) to -06 . . . . . . . . . . . . . . . . . 119 293 H.3.11. Changes from draft-ietf-emailcore-rfc5321bis-06 294 (2021-11-07) to -07 . . . . . . . . . . . . . . . . . 120 295 H.3.12. Changes from draft-ietf-emailcore-rfc5321bis-07 296 (2021-12-04) to -08 . . . . . . . . . . . . . . . . . 120 297 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 298 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 123 300 1. Introduction 302 1.1. Transport of Electronic Mail 304 The objective of the Simple Mail Transfer Protocol (SMTP) is to 305 transfer mail reliably and efficiently. 307 SMTP is independent of the particular transmission subsystem and 308 requires only a reliable ordered data stream channel. While this 309 document specifically discusses transport over TCP, other transports 310 are possible. Appendices to RFC 821 [3] describe some of them. 312 An important feature of SMTP is its capability to transport mail 313 across multiple networks, usually referred to as "SMTP mail relaying" 314 (see Section 3.6). A network consists of the mutually-TCP-accessible 315 hosts on the public Internet, the mutually-TCP-accessible hosts on a 316 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 317 environment utilizing a non-TCP transport-level protocol. Using 318 SMTP, a process can transfer mail to another process on the same 319 network or to some other network via a relay or gateway process 320 accessible to both networks. 322 In this way, a mail message may pass through a number of intermediate 323 relay or gateway hosts on its path from sender to ultimate recipient. 324 The Mail eXchanger mechanisms of the domain name system (RFC 1035 325 [4], RFC 974 [16], and Section 5 of this document) are used to 326 identify the appropriate next-hop destination for a message being 327 transported. 329 1.2. History and Context for This Document 331 This document is a specification of the basic protocol for the 332 Internet electronic mail transport. It consolidates, updates and 333 clarifies, but does not add new or change existing functionality of 334 the following: 336 * the original SMTP (Simple Mail Transfer Protocol) specification of 337 RFC 821 [3], 339 * domain name system requirements and implications for mail 340 transport from RFC 1035 [4] and RFC 974 [16], 342 * the clarifications and applicability statements in RFC 1123 [5], 344 * the new error codes added by RFC 1846 [20] and later by RFC 7504 345 [45], obsoleting both of those documents, and 347 * material drawn from the SMTP Extension mechanisms in RFC 1869 348 [22]. 350 It also includes editorial and clarification changes that were made 351 to RFC 2821 [30] to bring that specification to Draft Standard and 352 similar changes to RFC 5321 [47] to bring the current document to 353 Internet Standard. 355 It may help the reader to understand that, to reduce the risk of 356 introducing errors, large parts of the document essentially merge the 357 earlier specifications listed in the bullet points above rather than 358 providing a completely rewritten, reorganized, and integrated 359 description of SMTP. An index is provided to assist in the quest for 360 information. 362 It obsoletes RFCs 5321 [47] (the earlier version of this 363 specification), 1846 [20] and incorporates the substance of 7504 364 [45]7504 (specification of reply codes), and 7505 [46] (the "Null MX" 365 specification). 367 // JcK: 202107219: does the text that follows need rewriting? See 368 // comment in Abstract. 369 Although SMTP was designed as a mail transport and delivery protocol, 370 this specification also contains information that is important to its 371 use as a "mail submission" protocol, as recommended for Post Office 372 Protocol (POP) (RFC 937 [14], RFC 1939 [23]) and IMAP (RFC 3501 373 [36]). In general, the separate mail submission protocol specified 374 in RFC 6409 [41] is now preferred to direct use of SMTP; more 375 discussion of that subject appears in that document. 377 Section 2.3 provides definitions of terms specific to this document. 378 Except when the historical terminology is necessary for clarity, this 379 document uses the current 'client' and 'server' terminology to 380 identify the sending and receiving SMTP processes, respectively. 382 A companion document, RFC 5322 [12], discusses message header 383 sections and bodies and specifies formats and structures for them. 384 Other relevant documents and their relationships are discussed in a 385 forthcoming Applicability Statement [48]. 387 1.3. Document Conventions 389 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 390 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 391 document are to be interpreted as described in RFC 2119 [1]. As each 392 of these terms was intentionally and carefully chosen to improve the 393 interoperability of email, each use of these terms is to be treated 394 as a conformance requirement. 396 Because this document has a long history and to avoid the risk of 397 various errors and of confusing readers and documents that point to 398 this one, most examples and the domain names they contain are 399 preserved from RFC 2821. Readers are cautioned that these are 400 illustrative examples that should not actually be used in either code 401 or configuration files. 403 2. The SMTP Model 405 // [5321bis] [[Editor's Note: There have been extensive and repeated 406 // discussions on the SMTP and IETF lists about whether this document 407 // should say something about hop-by-hop (MTA-to-MTA) SMTP 408 // authentication and, if so, what?? Note that end to end message 409 // authentication is almost certainly out of scope for SMTP.]] Cf. 410 // Appendix G.7.17 412 2.1. Basic Structure 414 The SMTP design can be pictured as: 416 +----------+ +----------+ 417 +------+ | | | | 418 | User |<-->| | SMTP | | 419 +------+ | Client- |Commands/Replies| Server- | 420 +------+ | SMTP |<-------------->| SMTP | +------+ 421 | File |<-->| | and Mail | |<-->| File | 422 |System| | | | | |System| 423 +------+ +----------+ +----------+ +------+ 424 SMTP client SMTP server 426 When an SMTP client has a message to transmit, it establishes a two- 427 way transmission channel to an SMTP server. The responsibility of an 428 SMTP client is to transfer mail messages to one or more SMTP servers, 429 or report its failure to do so. 431 The means by which a mail message is presented to an SMTP client, and 432 how that client determines the identifier(s) ("names") of the 433 domain(s) to which mail messages are to be transferred, are local 434 matters. They are not addressed by this document. In some cases, 435 the designated domain(s), or those determined by an SMTP client, will 436 identify the final destination(s) of the mail message. In other 437 cases, common with SMTP clients associated with implementations of 438 the POP (RFC 937 [14], RFC 1939 [23]) or IMAP (RFC 3501 [36]) 439 protocols, or when the SMTP client is inside an isolated transport 440 service environment, the domain determined will identify an 441 intermediate destination through which all mail messages are to be 442 relayed. SMTP clients that transfer all traffic regardless of the 443 target domains associated with the individual messages, or that do 444 not maintain queues for retrying message transmissions that initially 445 cannot be completed, may otherwise conform to this specification but 446 are not considered fully-capable. Fully-capable SMTP 447 implementations, including the relays used by these less capable 448 ones, and their destinations, are expected to support all of the 449 queuing, retrying, and alternate address functions discussed in this 450 specification. In many situations and configurations, the less- 451 capable clients discussed above SHOULD be using the message 452 submission protocol (RFC 6409 [41]) rather than SMTP. 454 The means by which an SMTP client, once it has determined a target 455 domain, determines the identity of an SMTP server to which a copy of 456 a message is to be transferred, and then performs that transfer, are 457 covered by this document. To effect a mail transfer to an SMTP 458 server, an SMTP client establishes a two-way transmission channel to 459 that SMTP server. An SMTP client determines the address of an 460 appropriate host running an SMTP server by resolving a destination 461 domain name to either an intermediate Mail eXchanger host or a final 462 target host. 464 An SMTP server may be either the ultimate destination or an 465 intermediate "relay" (that is, it may assume the role of an SMTP 466 client after receiving the message) or "gateway" (that is, it may 467 transport the message further using some protocol other than SMTP). 468 SMTP commands are generated by the SMTP client and sent to the SMTP 469 server. SMTP replies are sent from the SMTP server to the SMTP 470 client in response to the commands. 472 In other words, message transfer can occur in a single connection 473 between the original SMTP-sender and the final SMTP-recipient, or can 474 occur in a series of hops through intermediary systems. In either 475 case, once the server has issued a success response at the end of the 476 mail data, a formal handoff of responsibility for the message occurs: 477 the protocol requires that a server MUST accept responsibility for 478 either delivering the message or properly reporting the failure to do 479 so (see Sections 6.1, 6.2, and 7.8, below). 481 Once the transmission channel is established and initial handshaking 482 is completed, the SMTP client normally initiates a mail transaction. 483 Such a transaction consists of a series of commands to specify the 484 originator and destination of the mail and transmission of the 485 message content (including any lines in the header section or other 486 structure) itself. When the same message is sent to multiple 487 recipients, this protocol encourages the transmission of only one 488 copy of the data for all recipients at the same destination (or 489 intermediate relay) host. 491 The server responds to each command with a reply; replies may 492 indicate that the command was accepted, that additional commands are 493 expected, or that a temporary or permanent error condition exists. 494 Commands specifying the sender or recipients may include server- 495 permitted SMTP service extension requests, as discussed in 496 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 497 although this can be modified by mutually agreed upon extension 498 requests such as command pipelining (RFC 2920 [31]). 500 Once a given mail message has been transmitted, the client may either 501 request that the connection be shut down or may initiate other mail 502 transactions. In addition, an SMTP client may use a connection to an 503 SMTP server for ancillary services such as verification of email 504 addresses or retrieval of mailing list subscriber addresses. 506 As suggested above, this protocol provides mechanisms for the 507 transmission of mail. Historically, this transmission normally 508 occurred directly from the sending user's host to the receiving 509 user's host when the two hosts are connected to the same transport 510 service. When they are not connected to the same transport service, 511 transmission occurs via one or more relay SMTP servers. A very 512 common case in the Internet today involves submission of the original 513 message to an intermediate, "message submission" server, which is 514 similar to a relay but has some additional properties; such servers 515 are discussed in Section 2.3.10 and at some length in RFC 6409 [41]. 516 An intermediate host that acts as either an SMTP relay or as a 517 gateway into some other transmission environment is usually selected 518 through the use of the domain name service (DNS) Mail eXchanger 519 mechanism. 521 2.2. The Extension Model 523 2.2.1. Background 525 In an effort that started in 1990, approximately a decade after RFC 526 821 was completed, the protocol was modified with a "service 527 extensions" model that permits the client and server to agree to 528 utilize shared functionality beyond the original SMTP requirements. 529 The SMTP extension mechanism defines a means whereby an extended SMTP 530 client and server may recognize each other, and the server can inform 531 the client as to the service extensions that it supports. 533 Contemporary SMTP implementations MUST support the basic extension 534 mechanisms. For instance, servers MUST support the EHLO command even 535 if they do not implement any specific extensions and clients SHOULD 536 preferentially utilize EHLO rather than HELO. (However, for 537 compatibility with older conforming implementations, SMTP clients and 538 servers MUST support the original HELO mechanisms as a fallback.) 539 Unless the different characteristics of HELO must be identified for 540 interoperability purposes, this document discusses only EHLO. 542 SMTP is widely deployed and high-quality implementations have proven 543 to be very robust. However, the Internet community now considers 544 some services to be important that were not anticipated when the 545 protocol was first designed. If support for those services is to be 546 added, it must be done in a way that permits older implementations to 547 continue working acceptably. The extension framework consists of: 549 * The SMTP command EHLO, superseding the earlier HELO, 551 * a registry of SMTP service extensions, 553 * additional parameters to the SMTP MAIL and RCPT commands, and 555 * optional replacements for commands defined in this protocol, such 556 as for DATA in non-ASCII transmissions (RFC 3030 [33]). 558 SMTP's strength comes primarily from its simplicity. Experience with 559 many protocols has shown that protocols with few options tend towards 560 ubiquity, whereas protocols with many options tend towards obscurity. 562 Each and every extension, regardless of its benefits, must be 563 carefully scrutinized with respect to its implementation, deployment, 564 and interoperability costs. In many cases, the cost of extending the 565 SMTP service will likely outweigh the benefit. 567 2.2.2. Definition and Registration of Extensions 569 The IANA maintains a registry of SMTP service extensions [52]. A 570 corresponding EHLO keyword value is associated with each extension. 571 Each service extension registered with the IANA must be defined in a 572 formal Standards-Track or IESG-approved Experimental protocol 573 document. The definition must include: 575 * the textual name of the SMTP service extension; 577 * the EHLO keyword value associated with the extension; 579 * the syntax and possible values of parameters associated with the 580 EHLO keyword value; 582 * any additional SMTP verbs associated with the extension 583 (additional verbs will usually be, but are not required to be, the 584 same as the EHLO keyword value); 586 * any new parameters the extension associates with the MAIL or RCPT 587 verbs; 589 * a description of how support for the extension affects the 590 behavior of a server and client SMTP; and 592 * the increment by which the extension is increasing the maximum 593 length of the commands MAIL and/or RCPT, over that specified in 594 this Standard. 596 Any keyword value presented in the EHLO response MUST correspond to a 597 Standard, Standards-Track, or IESG-approved Experimental SMTP service 598 extension registered with IANA. A conforming server MUST NOT offer 599 keyword values that are not described in a registered extension. 601 2.2.3. Special Issues with Extensions 603 Extensions that change fairly basic properties of SMTP operation are 604 permitted. The text in other sections of this document must be 605 understood in that context. In particular, extensions can change the 606 minimum limits specified in Section 4.5.3, can change the ASCII 607 character set requirement as mentioned above, or can introduce some 608 optional modes of message handling. 610 In particular, if an extension implies that the delivery path 611 normally supports special features of that extension, and an 612 intermediate SMTP system finds a next hop that does not support the 613 required extension, it MAY choose, based on the specific extension 614 and circumstances, to requeue the message and try later and/or try an 615 alternate MX host. If this strategy is employed, the timeout to fall 616 back to an unextended format (if one is available) SHOULD be less 617 than the normal timeout for bouncing as undeliverable (e.g., if 618 normal timeout is three days, the requeue timeout before attempting 619 to transmit the mail without the extension might be one day). 621 2.3. SMTP Terminology 623 2.3.1. Mail Objects 625 SMTP transports a mail object. A mail object contains an envelope 626 and content. 628 The SMTP envelope is sent as a series of SMTP protocol units 629 (described in Section 3). It consists of an originator address (to 630 which error reports should be directed), one or more recipient 631 addresses, and optional protocol extension material. Historically, 632 variations on the reverse-path (originator) address specification 633 command (MAIL) could be used to specify alternate delivery modes, 634 such as immediate display; those variations have now been deprecated 635 (see Appendix F and Appendix F.6). 637 The SMTP content is sent in the SMTP DATA protocol unit and has two 638 parts: the header section and the body. If the content conforms to 639 other contemporary standards, the header section consists of a 640 collection of header fields, each consisting of a header name, a 641 colon, and data, structured as in the message format specification 642 (RFC 5322 [12]); the body, if structured, is defined according to 643 MIME (RFC 2045 [25]). The content is textual in nature, expressed 644 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 645 "8BITMIME", RFC 6152 [44]) may relax this restriction for the content 646 body, the content header fields are always encoded using the US-ASCII 647 repertoire. Two MIME extensions (RFC 2047 [26] and RFC 2231 [29]) 648 define an algorithm for representing header values outside the US- 649 ASCII repertoire, while still encoding them using the US-ASCII 650 repertoire. 652 2.3.2. Senders and Receivers 654 In RFC 821, the two hosts participating in an SMTP transaction were 655 described as the "SMTP-sender" and "SMTP-receiver". This document 656 has been changed to reflect current industry terminology and hence 657 refers to them as the "SMTP client" (or sometimes just "the client") 658 and "SMTP server" (or just "the server"), respectively. Since a 659 given host may act both as server and client in a relay situation, 660 "receiver" and "sender" terminology is still used where needed for 661 clarity. 663 2.3.3. Mail Agents and Message Stores 665 Additional mail system terminology became common after RFC 821 was 666 published and, where convenient, is used in this specification. In 667 particular, SMTP servers and clients provide a mail transport service 668 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 669 Agents" (MUAs or UAs) are normally thought of as the sources and 670 targets of mail. At the source, an MUA might collect mail to be 671 transmitted from a user and hand it off to an MTA or, more commonly 672 in recent years, a specialized variation on an MTA called a 673 "Submission Server" (MSA) [41]. . At the other end of the process, 674 the final ("delivery") MTA would be thought of as handing the mail 675 off to an MUA (or at least transferring responsibility to it, e.g., 676 by depositing the message in a "message store"). However, while 677 these terms are used with at least the appearance of great precision 678 in other environments, the implied boundaries between MUAs and MTAs 679 often do not accurately match common, and conforming, practices with 680 Internet mail. Hence, the reader should be cautious about inferring 681 the strong relationships and responsibilities that might be implied 682 if these terms were used elsewhere 684 2.3.4. Host 686 For the purposes of this specification, a host is a computer system 687 attached to the Internet (or, in some cases, to a private TCP/IP 688 network) and supporting the SMTP protocol. Hosts are known by names 689 (see the next section); they SHOULD NOT be identified by numerical 690 addresses, i.e., by address literals as described in Section 4.1.2. 692 2.3.5. Domain Names 694 A domain name (or often just a "domain") consists of one or more 695 components, separated by dots if more than one appears. In the case 696 of a top-level domain used by itself in an email address, a single 697 string is used without any dots. This makes the requirement, 698 described in more detail below, that only fully-qualified domain 699 names appear in SMTP transactions on the public Internet, 700 particularly important where top-level domains are involved. These 701 components ("labels" in the DNS terminology of RFC 1035 [4]) are 702 restricted for purposes of SMTP as defined here to consist of a 703 sequence of letters, digits, and hyphens drawn from the ASCII 704 character set [2] and conforming to what RFC 1035 Section 2.3.1 calls 705 the "preferred name syntax". Domain names are used as names of hosts 706 and, except where additionally restricted in this document, of other 707 entities in the domain name hierarchy. For example, a domain may 708 refer to a host alias (label of a CNAME RR) or the label of Mail 709 eXchanger records to be used to deliver mail instead of representing 710 a host name. See RFC 1035 and Section 5 of this specification. 712 The domain name, as described in this document and in RFC 1035 [4], 713 MUST be the entire, fully-qualified name (often referred to as an 714 "FQDN"). Other than an address literal (see Section 4.1.3) where 715 those are permitted, any string that is not a domain name in FQDN 716 form is no more than a reference to be interpreted locally. Such 717 local references for domain names MUST NOT appear in any SMTP 718 transaction (Cf. Section 5). Mechanisms for inferring FQDNs from 719 local references (including partial names or local aliases) are 720 outside of this specification and normally the province of message 721 submission. Due to a history of problems, SMTP servers used for 722 initial submission of messages SHOULD NOT make such inferences 723 (Message Submission Servers [41] have somewhat more flexibility) and 724 intermediate (relay) SMTP servers MUST NOT make them. 726 // [rfc5321 Editor's Note 20211231] The sentence starting with 727 // "Mechanisms" and the one immediately following it above moved from 728 // Section 5.1, but perhaps they should be dropped entirely and/or 729 // elaborated on in the A/S. 731 When domain names are used in SMTP, and unless further restricted in 732 this document, names that can be resolved to MX RRs or address (i.e., 733 A or AAAA) RRs (as discussed in Section 5) are permitted, as are 734 CNAME RRs whose targets can be resolved, in turn, to MX or address 735 RRs. There are two exceptions to the rule requiring FQDNs: 737 * The domain name given in the EHLO command MUST be either a primary 738 host name (a domain name that resolves to an address RR) or, if 739 the host has no name, an address literal, as described in 740 Section 4.1.3 and discussed further in the EHLO discussion of 741 Section 4.1.4. 743 * The reserved mailbox name "postmaster" may be used in a RCPT 744 command without domain qualification (see Section 4.1.1.3) and 745 MUST be accepted if so used. 747 2.3.6. Buffer and State Table 749 SMTP sessions are stateful, with both parties carefully maintaining a 750 common view of the current state. In this document, we model this 751 state by a virtual "buffer" and a "state table" on the server that 752 may be used by the client to, for example, "clear the buffer" or 753 "reset the state table", causing the information in the buffer to be 754 discarded and the state to be returned to some previous state. 756 2.3.7. Commands and Replies 758 SMTP commands and, unless altered by a service extension, message 759 data, are transmitted from the sender to the receiver via the 760 transmission channel in "lines". 762 An SMTP reply is an acknowledgment (positive or negative) sent in 763 "lines" from receiver to sender via the transmission channel in 764 response to a command. The general form of a reply is a numeric 765 completion code (indicating failure or success) usually followed by a 766 text string. The codes are for use by programs and the text is 767 usually intended for human users. RFC 3463 [7], specifies further 768 structuring of the reply strings, including the use of supplemental 769 and more specific completion codes (see also RFC 5248 [43]). 771 2.3.8. Lines 773 Lines consist of zero or more data characters terminated by the 774 sequence ASCII character "CR" (hex value 0D) followed immediately by 775 ASCII character "LF" (hex value 0A). This termination sequence is 776 denoted as in this document. Conforming implementations MUST 777 NOT recognize or generate any other character or character sequence 778 as a line terminator. Limits MAY be imposed on line lengths by 779 servers (see Section 4). 781 In addition, the appearance of "bare" "CR" or "LF" characters in text 782 (i.e., either without the other) has a long history of causing 783 problems in mail implementations and applications that use the mail 784 system as a tool. SMTP client implementations MUST NOT transmit 785 these characters except when they are intended as line terminators 786 and then MUST, as indicated above, transmit them only as a 787 sequence. 789 2.3.9. Message Content and Mail Data 791 The terms "message content" and "mail data" are used interchangeably 792 in this document to describe the material transmitted after the DATA 793 command is accepted and before the end of data indication is 794 transmitted. Message content includes the message header section and 795 the possibly structured message body. In the absence of extensions, 796 both are required to be ASCII (see Section 2.3.1). The MIME 797 specification (RFC 2045 [25]) provides the standard mechanisms for 798 structured message bodies. 800 2.3.10. Originator, Delivery, Relay, and Gateway Systems 802 This specification makes a distinction among four types of SMTP 803 systems, based on the role those systems play in transmitting 804 electronic mail. An "originating" system (sometimes called an SMTP 805 originator) introduces mail into the Internet or, more generally, 806 into a transport service environment. A "delivery" SMTP system is 807 one that receives mail from a transport service environment and 808 passes it to a mail user agent or deposits it in a message store that 809 a mail user agent is expected to subsequently access. A "relay" SMTP 810 system (usually referred to just as a "relay") receives mail from an 811 SMTP client and transmits it, without modification to the message 812 data other than adding trace information (see Section 4.4), to 813 another SMTP server for further relaying or for delivery. 815 A "gateway" SMTP system (usually referred to just as a "gateway") 816 receives mail from a client system in one transport environment and 817 transmits it to a server system in another transport environment. 818 Differences in protocols or message semantics between the transport 819 environments on either side of a gateway may require that the gateway 820 system perform transformations to the message that are not permitted 821 to SMTP relay systems. For the purposes of this specification, 822 firewalls that rewrite addresses should be considered as gateways, 823 even if SMTP is used on both sides of them (see RFC 2979 [32]). 824 // [5321bis] [[Note in draft/Placeholder: There has been a request to 825 // expand this section, possibly into a more extensive model of 826 // Internet mail. Comments from others solicited. In particular, 827 // does RFC 5598 make that suggestion OBE?]] 829 2.3.11. Mailbox and Address 831 As used in this specification, an "address" is a character string 832 that identifies a user to whom mail will be sent or a location into 833 which mail will be deposited. The term "mailbox" refers to that 834 depository. The two terms are typically used interchangeably unless 835 the distinction between the location in which mail is placed (the 836 mailbox) and a reference to it (the address) is important. An 837 address normally consists of user and domain specifications. The 838 standard mailbox naming convention is defined to be "local- 839 part@domain"; contemporary usage permits a much broader set of 840 applications than simple "user names". Consequently, and due to a 841 long history of problems when intermediate hosts have attempted to 842 optimize transport by modifying them, the local-part MUST be 843 interpreted and assigned semantics only by the host specified in the 844 domain part of the address. 846 2.4. General Syntax Principles and Transaction Model 848 SMTP commands and replies have a rigid syntax. All commands begin 849 with a command verb. All replies begin with a three digit numeric 850 code. In some commands and replies, arguments are required following 851 the verb or reply code. Some commands do not accept arguments (after 852 the verb), and some reply codes are followed, sometimes optionally, 853 by free form text. In both cases, where text appears, it is 854 separated from the verb or reply code by a space character. Complete 855 definitions of commands and replies appear in Section 4. 857 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 858 and extension name keywords) are not case sensitive, with the sole 859 exception in this specification of a mailbox local-part (SMTP 860 Extensions may explicitly specify case-sensitive elements). That is, 861 a command verb, an argument value other than a mailbox local-part, 862 and free form text MAY be encoded in upper case, lower case, or any 863 mixture of upper and lower case with no impact on its meaning. The 864 local-part of a mailbox MUST BE treated as case sensitive. 865 Therefore, SMTP implementations MUST take care to preserve the case 866 of mailbox local-parts. In particular, for some hosts, the user 867 "smith" is different from the user "Smith". However, exploiting the 868 case sensitivity of mailbox local-parts impedes interoperability and 869 is discouraged. Mailbox domains follow normal DNS rules and are 870 hence not case sensitive. 872 A few SMTP servers, in violation of this specification (and RFC 821) 873 require that command verbs be encoded by clients in upper case. 874 Implementations MAY wish to employ this encoding to accommodate those 875 servers. 877 The argument clause consists of a variable-length character string 878 ending with the end of the line, i.e., with the character sequence 879 . The receiver will take no action until this sequence is 880 received. 882 The syntax for each command is shown with the discussion of that 883 command. Common elements and parameters are shown in Section 4.1.2. 885 Commands and replies are composed of characters from the ASCII 886 character set [2]. When the transport service provides an 8-bit byte 887 (octet) transmission channel, each 7-bit character is transmitted, 888 right justified, in an octet with the high-order bit cleared to zero. 889 More specifically, the unextended SMTP service provides 7-bit 890 transport only. An originating SMTP client that has not successfully 891 negotiated an appropriate extension with a particular server (see the 892 next paragraph) MUST NOT transmit messages with information in the 893 high-order bit of octets. If such messages are transmitted in 894 violation of this rule, receiving SMTP servers MAY clear the high- 895 order bit or reject the message as invalid. In general, a relay SMTP 896 SHOULD assume that the message content it has received is valid and, 897 assuming that the envelope permits doing so, relay it without 898 inspecting that content. Of course, if the content is mislabeled and 899 the data path cannot accept the actual content, this may result in 900 the ultimate delivery of a severely garbled message to the recipient. 901 Delivery SMTP systems MAY reject such messages, or return them as 902 undeliverable, rather than deliver them. In the absence of a server- 903 offered extension explicitly permitting it, a sending SMTP system is 904 not permitted to send envelope commands in any character set other 905 than US-ASCII. Receiving systems SHOULD reject such commands, 906 normally using "500 syntax error - invalid character" replies. 908 8-bit message content transmission MAY be requested of the server by 909 a client using extended SMTP facilities, notably the "8BITMIME" 910 extension, RFC 6152 [44]. 8BITMIME SHOULD be supported by SMTP 911 servers. However, it MUST NOT be construed as authorization to 912 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 913 transmission of any envelope material in other than ASCII. 8BITMIME 914 MUST NOT be requested by senders for material with the high bit on 915 that is not in MIME format with an appropriate content-transfer 916 encoding; servers MAY reject such messages. 918 The metalinguistic notation used in this document corresponds to the 919 "Augmented BNF" used in other Internet mail system documents. The 920 reader who is not familiar with that syntax should consult the ABNF 921 specification in RFC 5234 [11]. Metalanguage terms used in running 922 text are surrounded by pointed brackets (e.g., ) for clarity. 923 The reader is cautioned that the grammar expressed in the 924 metalanguage is not comprehensive. There are many instances in which 925 provisions in the text constrain or otherwise modify the syntax or 926 semantics implied by the grammar. 928 3. The SMTP Procedures: An Overview 930 This section contains descriptions of the procedures used in SMTP: 931 session initiation, mail transaction, forwarding mail, verifying 932 mailbox names and expanding mailing lists, and opening and closing 933 exchanges. Comments on relaying, a note on mail domains, and a 934 discussion of changing roles are included at the end of this section. 935 Several complete scenarios are presented in Appendix D. 937 3.1. Session Initiation 939 An SMTP session is initiated when a client opens a connection to a 940 server and the server responds with an opening message. 942 SMTP server implementations MAY include identification of their 943 software and version information in the connection greeting reply 944 after the 220 code, a practice that permits more efficient isolation 945 and repair of any problems. Implementations MAY make provision for 946 SMTP servers to disable the software and version announcement where 947 it causes security concerns. While some systems also identify their 948 contact point for mail problems, this is not a substitute for 949 maintaining the required "postmaster" address (see Section 4). 951 The SMTP protocol allows a server to formally reject a mail session 952 while still allowing the initial connection as follows: a 521 953 response MAY be given in the initial connection opening message 954 instead of the 220. A server taking this approach MUST still wait 955 for the client to send a QUIT (see Section 4.1.1.10) before closing 956 the connection and SHOULD respond to any intervening commands with 957 "503 bad sequence of commands". Since an attempt to make an SMTP 958 connection to such a system is probably in error, a server returning 959 a 521 961 // (or 554?) 962 response on connection opening SHOULD provide enough information in 963 the reply text to facilitate debugging of the sending system. See 964 Section 4.2.4.2. 966 3.2. Client Initiation 968 Once the server has sent the greeting (welcoming) message and the 969 client has received it, the client normally sends the EHLO command to 970 the server, indicating the client's identity. In addition to opening 971 the session, use of EHLO indicates that the client is able to process 972 service extensions and requests that the server provide a list of the 973 extensions it supports. Older SMTP systems that are unable to 974 support service extensions, and contemporary clients that do not 975 require service extensions in the mail session being initiated, MAY 976 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 977 style response to a HELO command. For a particular connection 978 attempt, if the server returns a "command not recognized" response to 979 EHLO, the client SHOULD be able to fall back and send HELO. 981 In the EHLO command, the host sending the command identifies itself; 982 the command may be interpreted as saying "Hello, I am " (and, 983 in the case of EHLO, "and I support service extension requests"). 985 3.3. Mail Transactions 987 There are three steps to SMTP mail transactions. The transaction 988 starts with a MAIL command that gives the sender identification. (In 989 general, the MAIL command may be sent only when no mail transaction 990 is in progress; see Section 4.1.4.) A series of one or more RCPT 991 commands follows, giving the receiver information. Then, a DATA 992 command initiates transfer of the mail data and is terminated by the 993 "end of mail" data indicator, which also confirms (and terminates) 994 the transaction. 996 Mail transactions are also terminated by the RSET command 997 (Section 4.1.1.5), the sending of an EHLO command (Section 3.2), or 998 the sending of a QUIT command (Section 3.8) which terminates both any 999 active mail transaction and the SMTP connection. 1001 The first step in the procedure is the MAIL command. 1003 MAIL FROM: [SP ] 1005 This command tells the SMTP-receiver that a new mail transaction is 1006 starting and to reset all its state tables and buffers, including any 1007 recipients or mail data. The portion of the first or 1008 only argument contains the source mailbox (between "<" and ">" 1009 brackets), which can be used to report errors (see Section 4.2 for a 1010 discussion of error reporting). If accepted, the SMTP server returns 1011 a "250 OK" reply. If the mailbox specification is not acceptable for 1012 some reason, the server MUST return a reply indicating whether the 1013 failure is permanent (i.e., will occur again if the client tries to 1014 send the same address again) or temporary (i.e., the address might be 1015 accepted if the client tries again later). Despite the apparent 1016 scope of this requirement, there are circumstances in which the 1017 acceptability of the reverse-path may not be determined until one or 1018 more forward-paths (in RCPT commands) can be examined. In those 1019 cases, the server MAY reasonably accept the reverse-path (with a 250 1020 reply) and then report problems after the forward-paths are received 1021 and examined. Normally, failures produce 550 or 553 replies. 1023 Historically, the was permitted to contain more than 1024 just a mailbox; however source routing is now deprecated (see 1025 Appendix F.2). 1027 The optional are associated with negotiated SMTP 1028 service extensions (see Section 2.2). 1030 The second step in the procedure is the RCPT command. This step of 1031 the procedure can be repeated any number of times. 1033 RCPT TO: [ SP ] 1035 The first or only argument to this command includes a forward-path 1036 (normally a mailbox local-part and domain, always surrounded by "<" 1037 and ">" brackets) identifying one recipient. If accepted, the SMTP 1038 server returns a "250 OK" reply and stores the forward-path. If the 1039 recipient is known not to be a deliverable address, the SMTP server 1040 returns a 550 reply, typically with a string such as "no such user - 1041 " and the mailbox name (other circumstances and reply codes are 1042 possible). 1044 Historically, the was permitted to contain a source 1045 routing list of hosts and the destination mailbox; however, source 1046 routes are now deprecated (see Appendix F.2). Restricted-capability 1047 clients MUST NOT assume that any SMTP server on the Internet can be 1048 used as their mail processing (relaying) site. If a RCPT command 1049 appears without a previous MAIL command, the server MUST return a 503 1050 "Bad sequence of commands" response. The optional 1051 are associated with negotiated SMTP service extensions (see 1052 Section 2.2). 1053 // [5321bis]: this section would be improved by being more specific 1054 // about where mail transactions begin and end and then talking about 1055 // "transaction state" here, rather than specific prior commands. 1056 // --JcK 1058 Since it has been a common source of errors, it is worth noting that 1059 spaces are not permitted on either side of the colon following FROM 1060 in the MAIL command or TO in the RCPT command. The syntax is exactly 1061 as given above. 1063 The third step in the procedure is the DATA command (or some 1064 alternative specified in a service extension). 1066 DATA 1068 If accepted, the SMTP server returns a 354 Intermediate reply and 1069 considers all succeeding lines up to but not including the end of 1070 mail data indicator to be the message text. When the end of text is 1071 successfully received and stored, the SMTP-receiver sends a "250 OK" 1072 reply. 1074 Since the mail data is sent on the transmission channel, the end of 1075 mail data must be indicated so that the command and reply dialog can 1076 be resumed. SMTP indicates the end of the mail data by sending a 1077 line containing only a "." (period or full stop, hex 2E). A 1078 transparency procedure is used to prevent this from interfering with 1079 the user's text (see Section 4.5.2). 1081 The end of mail data indicator also confirms the mail transaction and 1082 tells the SMTP server to now process the stored recipients and mail 1083 data. If accepted, the SMTP server returns a "250 OK" reply. The 1084 DATA command can fail at only two points in the protocol exchange: 1086 If there was no MAIL, or no RCPT, command, or all such commands were 1087 rejected, the server MAY return a "command out of sequence" (503) or 1088 "no valid recipients" (554) reply in response to the DATA command. 1089 If one of those replies (or any other 5yz reply) is received, the 1090 client MUST NOT send the message data; more generally, message data 1091 MUST NOT be sent unless a 354 reply is received. 1093 If the verb is initially accepted and the 354 reply issued, the DATA 1094 command should fail only if the mail transaction was incomplete (for 1095 example, no recipients), if resources were unavailable (including, of 1096 course, the server unexpectedly becoming unavailable), or if the 1097 server determines that the message should be rejected for policy or 1098 other reasons. 1100 However, in practice, some servers do not perform recipient 1101 verification until after the message text is received. These servers 1102 SHOULD treat a failure for one or more recipients as a "subsequent 1103 failure" and return a mail message as discussed in Section 6 and, in 1104 particular, in Section 6.1. Using a "550 mailbox not found" (or 1105 equivalent) reply code after the data are accepted makes it difficult 1106 or impossible for the client to determine which recipients failed. 1108 When the RFC 822 format ([13], [12]) is being used, the mail data 1109 include the header fields such as those named Date, Subject, To, Cc, 1110 and From. Server SMTP systems SHOULD NOT reject messages based on 1111 perceived defects in the RFC 822 or MIME (RFC 2045 [25]) message 1112 header section or message body. In particular, they MUST NOT reject 1113 messages in which the numbers of Resent-header fields do not match or 1114 Resent-to appears without Resent-from and/or Resent-date. 1116 Mail transaction commands MUST be used in the order discussed above. 1118 3.4. Address Modification and Expansion 1120 3.4.1. Forwarding for Address Correction or Updating 1122 Forwarding support is most often required to consolidate and simplify 1123 addresses within, or relative to, some enterprise and less frequently 1124 to establish addresses to link a person's prior address with a 1125 current one. Silent forwarding of messages (without server 1126 notification to the sender), for security or non-disclosure purposes, 1127 is common in the contemporary Internet. 1129 In both the enterprise and the "new address" cases, information 1130 hiding (and sometimes security) considerations argue against exposure 1131 of the "final" address through the SMTP protocol as a side effect of 1132 the forwarding activity. This may be especially important when the 1133 final address may not even be reachable by the sender. Consequently, 1134 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1135 especially the 251 (corrected destination) and 551 reply codes from 1136 RCPT must be evaluated carefully by implementers and, when they are 1137 available, by those configuring systems (see also Section 7.4). 1139 In particular: 1141 * Servers MAY forward messages when they are aware of an address 1142 change. When they do so, they MAY either provide address-updating 1143 information with a 251 code, or may forward "silently" and return 1144 a 250 code. However, if a 251 code is used, they MUST NOT assume 1145 that the client will actually update address information or even 1146 return that information to the user. 1148 Alternately, 1150 * Servers MAY reject messages or return them as non-deliverable when 1151 they cannot be delivered precisely as addressed. When they do so, 1152 they MAY either provide address-updating information with a 551 1153 code, or may reject the message as undeliverable with a 550 code 1154 and no address-specific information. However, if a 551 code is 1155 used, they MUST NOT assume that the client will actually update 1156 address information or even return that information to the user. 1158 SMTP server implementations that support the 251 and/or 551 reply 1159 codes SHOULD provide configuration mechanisms so that sites that 1160 conclude that they would undesirably disclose information can disable 1161 or restrict their use. See Section 7.4 for further discussion of 1162 that issue. 1164 3.4.2. Aliases and Mailing Lists 1166 // [5321bis] If "alias and list models" are explained elsewhere, 1167 // cross reference. Also note that this section appears to prohibit 1168 // an exploder from adding List-* headers. That needs to be explicit 1169 // or finessed. 1170 An SMTP-capable host SHOULD support both the alias and the list 1171 models of address expansion for multiple delivery. When a message is 1172 delivered or forwarded to each address of an expanded list form, the 1173 return address in the envelope ("MAIL FROM:") MUST be changed to be 1174 the address of a person or other entity who administers the list. 1175 However, in this case, the message header section (RFC 5322 [12]) 1176 MUST be left unchanged; in particular, the "From" field of the header 1177 section is unaffected. 1179 An important mail facility is a mechanism for multi-destination 1180 delivery of a single message, by transforming (or "expanding" or 1181 "exploding") a pseudo-mailbox address into a list of destination 1182 mailbox addresses. When a message is sent to such a pseudo-mailbox 1183 (sometimes called an "exploder"), copies are forwarded or 1184 redistributed to each mailbox in the expanded list. Servers SHOULD 1185 simply utilize the addresses on the list; application of heuristics 1186 or other matching rules to eliminate some addresses, such as that of 1187 the originator, is strongly discouraged. We classify such a pseudo- 1188 mailbox as an "alias" or a "list", depending upon the expansion 1189 rules. 1191 3.4.2.1. Simple Aliases 1193 To expand an alias, the recipient mailer simply replaces the pseudo- 1194 mailbox address in the envelope with each of the expanded addresses 1195 in turn; the rest of the envelope and the message body are left 1196 unchanged. The message is then delivered or forwarded to each 1197 expanded address. 1199 3.4.2.2. Mailing Lists 1201 Processing of a mailing list may be said to operate by 1202 "redistribution" rather than by "forwarding" (as in the simple alias 1203 case in the subsection above). To expand a list, the recipient 1204 mailer replaces the pseudo-mailbox address in the envelope with each 1205 of the expanded addresses in turn. The return (backward-pointing) 1206 address in the envelope is changed so that all error messages 1207 generated by the final deliveries will be returned to a list 1208 administrator, not to the message originator, who generally has no 1209 control over the contents of the list and will typically find error 1210 messages annoying. Note that the key difference between handling 1211 simple aliases Section 3.4.2.1 and redistribution (this subsection) 1212 is the change to the backward-pointing address. When a system 1213 managing a list constrains its processing to the very limited set of 1214 modifications and actions described here, it is acting as part of an 1215 MTA; such list processing, like alias processing, can be treated as a 1216 continuation of email transit. 1218 Mailing list management systems do exist that perform additional, 1219 sometimes extensive, modifications to a message and its envelope. 1220 Such mailing lists need to be viewed as MUAs that accept a message 1221 delivery and then submit a new message for multiple recipients. 1223 3.5. Commands for Debugging Addresses 1225 3.5.1. Overview 1227 SMTP provides commands to verify a user name or obtain the content of 1228 a mailing list. This is done with the VRFY and EXPN commands, which 1229 have character string arguments. Implementations SHOULD support VRFY 1230 and EXPN (however, see Section 3.5.2 and Section 7.3). 1232 For the VRFY command, the string is a user name or a user name and 1233 domain (see below). If a normal (i.e., 250) response is returned, 1234 the response MAY include the full name of the user and MUST include 1235 the mailbox of the user. It MUST be in either of the following 1236 forms: 1238 User Name 1239 local-part@domain 1241 When a name that is the argument to VRFY could identify more than one 1242 mailbox, the server MAY either note the ambiguity or identify the 1243 alternatives. In other words, any of the following are legitimate 1244 responses to VRFY: 1246 553 User ambiguous 1248 or 1250 553- Ambiguous; Possibilities are 1251 553-Joe Smith 1252 553-Harry Smith 1253 553 Melvin Smith 1255 or 1257 553-Ambiguous; Possibilities 1258 553- 1259 553- 1260 553 1262 Under normal circumstances, a client receiving a 553 reply would be 1263 expected to expose the result to the user. Use of exactly the forms 1264 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1265 supplemented by extended reply codes, such as those described in RFC 1266 3463 [7], will facilitate automated translation into other languages 1267 as needed. Of course, a client that was highly automated or that was 1268 operating in another language than English might choose to try to 1269 translate the response to return some other indication to the user 1270 than the literal text of the reply, or to take some automated action 1271 such as consulting a directory service for additional information 1272 before reporting to the user. 1274 For the EXPN command, the string identifies a mailing list, and the 1275 successful (i.e., 250) multiline response MAY include the full name 1276 of the users and MUST give the mailboxes on the mailing list. 1278 In some hosts, the distinction between a mailing list and an alias 1279 for a single mailbox is a bit fuzzy, since a common data structure 1280 may hold both types of entries, and it is possible to have mailing 1281 lists containing only one mailbox. If a request is made to apply 1282 VRFY to a mailing list, a positive response MAY be given if a message 1283 so addressed would be delivered to everyone on the list, otherwise an 1284 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1285 user" or "252 Unable to verify members of mailing list"). If a 1286 request is made to expand a user name, the server MAY return a 1287 positive response consisting of a list containing one name, or an 1288 error MAY be reported (e.g., "550 That is a user name, not a mailing 1289 list"). 1291 In the case of a successful multiline reply (normal for EXPN), 1292 exactly one mailbox is to be specified on each line of the reply. 1293 The case of an ambiguous request is discussed above. 1295 "User name" is a fuzzy term and has been used deliberately. An 1296 implementation of the VRFY or EXPN commands MUST include at least 1297 recognition of local mailboxes as "user names". However, since 1298 current Internet practice often results in a single host handling 1299 mail for multiple domains, hosts, especially hosts that provide this 1300 functionality, SHOULD accept the "local-part@domain" form as a "user 1301 name"; hosts MAY also choose to recognize other strings as "user 1302 names". 1304 The case of expanding a mailbox list requires a multiline reply, such 1305 as: 1307 C: EXPN Example-People 1308 S: 250-Jon Postel 1309 S: 250-Fred Fonebone 1310 S: 250 Sam Q. Smith 1312 or 1314 C: EXPN Executive-Washroom-List 1315 S: 550 Access Denied to You. 1317 The character string arguments of the VRFY and EXPN commands cannot 1318 be further restricted due to the variety of implementations of the 1319 user name and mailbox list concepts. On some systems, it may be 1320 appropriate for the argument of the EXPN command to be a file name 1321 for a file containing a mailing list, but again there are a variety 1322 of file naming conventions in the Internet. Similarly, historical 1323 variations in what is returned by these commands are such that the 1324 response SHOULD be interpreted very carefully, if at all, and SHOULD 1325 generally only be used for diagnostic purposes. 1327 3.5.2. VRFY Normal Response 1329 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1330 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1332 domain name. In circumstances exceptional enough to justify 1333 violating the intent of this specification, free-form text MAY be 1334 returned. In order to facilitate parsing by both computers and 1335 people, addresses SHOULD appear in pointed brackets. When addresses, 1336 rather than free-form debugging information, are returned, EXPN and 1337 VRFY MUST return only valid domain addresses that are usable in SMTP 1338 RCPT commands. Consequently, if an address implies delivery to a 1339 program or other system, the mailbox name used to reach that target 1340 MUST be given. Paths (explicit source routes) MUST NOT be returned 1341 by VRFY or EXPN. 1343 Server implementations SHOULD support both VRFY and EXPN. For 1344 security reasons, implementations MAY provide local installations a 1345 way to disable either or both of these commands through configuration 1346 options or the equivalent (see Section 7.3). When these commands are 1347 supported, they are not required to work across relays when relaying 1348 is supported. Since they were both optional in RFC 821, but VRFY was 1349 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1350 listed as a service extension in an EHLO response. VRFY MAY be 1351 listed as a convenience but, since support for it is required, SMTP 1352 clients are not required to check for its presence on the extension 1353 list before using it. 1355 3.5.3. Meaning of VRFY or EXPN Success Response 1357 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1358 command unless it has actually verified the address. In particular, 1359 a server MUST NOT return 250 if all it has done is to verify that the 1360 syntax given is valid. In that case, 502 (Command not implemented) 1361 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1362 stated elsewhere, implementation (in the sense of actually validating 1363 addresses and returning information) of VRFY and EXPN are strongly 1364 recommended. Hence, implementations that return 500 or 502 for VRFY 1365 are not in full compliance with this specification. 1367 There may be circumstances where an address appears to be valid but 1368 cannot reasonably be verified in real time, particularly when a 1369 server is acting as a mail exchanger for another server or domain. 1370 "Apparent validity", in this case, would normally involve at least 1371 syntax checking and might involve verification that any domains 1372 specified were ones to which the host expected to be able to relay 1373 mail. In these situations, reply code 252 SHOULD be returned. These 1374 cases parallel the discussion of RCPT verification in Section 2.1. 1375 Similarly, the discussion in Section 3.4.1 applies to the use of 1376 reply codes 251 and 551 with VRFY (and EXPN) to indicate addresses 1377 that are recognized but that would be forwarded or rejected were mail 1378 received for them. Implementations generally SHOULD be more 1379 aggressive about address verification in the case of VRFY than in the 1380 case of RCPT, even if it takes a little longer to do so. 1382 3.5.4. Semantics and Applications of EXPN 1384 EXPN is often very useful in debugging and understanding problems 1385 with mailing lists and multiple-target-address aliases. Some systems 1386 have attempted to use source expansion of mailing lists as a means of 1387 eliminating duplicates. The propagation of aliasing systems with 1388 mail on the Internet for hosts (typically with MX and CNAME DNS 1389 records), for mailboxes (various types of local host aliases), and in 1390 various proxying arrangements has made it nearly impossible for these 1391 strategies to work consistently, and mail systems SHOULD NOT attempt 1392 them. 1394 3.6. Relaying and Mail Routing 1395 3.6.1. Mail eXchange Records and Relaying 1397 A relay SMTP server is usually the target of a DNS MX record that 1398 designates it, rather than the final delivery system. The relay 1399 server may accept or reject the task of relaying the mail in the same 1400 way it accepts or rejects mail for a local user. If it accepts the 1401 task, it then becomes an SMTP client, establishes a transmission 1402 channel to the next SMTP server specified in the DNS (according to 1403 the rules in Section 5), and sends it the mail. If it declines to 1404 relay mail to a particular address for policy reasons, a 550 response 1405 SHOULD be returned. 1407 This specification does not deal with the verification of return 1408 paths. Server efforts to verify a return path and actions to be 1409 taken under various circumstances are outside the scope of this 1410 specification. 1412 3.6.2. Message Submission Servers as Relays 1414 Many mail-sending clients exist, especially in conjunction with 1415 facilities that receive mail via POP3 or IMAP, that have limited 1416 capability to support some of the requirements of this specification, 1417 such as the ability to queue messages for subsequent delivery 1418 attempts. For these clients, it is common practice to make private 1419 arrangements to send all messages to a single server for processing 1420 and subsequent distribution. SMTP, as specified here, is not ideally 1421 suited for this role. A standardized mail submission protocol has 1422 been developed that is gradually superseding practices based on SMTP 1423 (see RFC 6409 [41]). In any event, because these arrangements are 1424 private and fall outside the scope of this specification, they are 1425 not described here. 1427 It is important to note that MX records can point to SMTP servers 1428 that act as gateways into other environments, not just SMTP relays 1429 and final delivery systems; see Sections 3.7 and 5. 1431 If an SMTP server has accepted the task of relaying the mail and 1432 later finds that the destination is incorrect or that the mail cannot 1433 be delivered for some other reason, then it MUST construct an 1434 "undeliverable mail" notification message and send it to the 1435 originator of the undeliverable mail (as indicated by the reverse- 1436 path). Formats specified for non-delivery reports by other standards 1437 (see, for example, RFC 3461 [34] and RFC 3464 [35]) SHOULD be used if 1438 possible. 1440 This notification message must be from the SMTP server at the relay 1441 host or the host that first determines that delivery cannot be 1442 accomplished. Of course, SMTP servers MUST NOT send notification 1443 messages about problems transporting notification messages. One way 1444 to prevent loops in error reporting is to specify a null reverse-path 1445 in the MAIL command of a notification message. When such a message 1446 is transmitted, the reverse-path MUST be set to null (see 1447 Section 4.5.5 for additional discussion). A MAIL command with a null 1448 reverse-path appears as follows: 1450 MAIL FROM:<> 1452 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1453 act upon the header section or body of the message data and MUST NOT 1454 do so except to add its own "Received:" header field (Section 4.4.1 1455 and possibly other trace header fields) and, optionally, to attempt 1456 to detect looping in the mail system (see Section 6.3). Of course, 1457 this prohibition also applies to any modifications of these header 1458 fields or text (see also Section 7.9). 1460 3.7. Mail Gatewaying 1462 While the relay function discussed above operates within the Internet 1463 SMTP transport service environment, MX records or various forms of 1464 explicit routing may require that an intermediate SMTP server perform 1465 a translation function between one transport service and another. As 1466 discussed in Section 2.3.10, when such a system is at the boundary 1467 between two transport service environments, we refer to it as a 1468 "gateway" or "gateway SMTP". 1470 Gatewaying mail between different mail environments, such as 1471 different mail formats and protocols, is complex and does not easily 1472 yield to standardization. However, some general requirements may be 1473 given for a gateway between the Internet and another mail 1474 environment. 1476 3.7.1. Header Fields in Gatewaying 1478 Header fields MAY be rewritten when necessary as messages are 1479 gatewayed across mail environment boundaries. This may involve 1480 inspecting the message body or interpreting the local-part of the 1481 destination address in spite of the prohibitions in Section 6.4. 1483 Other mail systems gatewayed to the Internet often use a subset of 1484 the RFC 822 header section or provide similar functionality with a 1485 different syntax, but some of these mail systems do not have an 1486 equivalent to the SMTP envelope. Therefore, when a message leaves 1487 the Internet environment, it may be necessary to fold the SMTP 1488 envelope information into the message header section. A possible 1489 solution would be to create new header fields to carry the envelope 1490 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1491 would require changes in mail programs in foreign environments and 1492 might risk disclosure of private information (see Section 7.2). 1494 3.7.2. Received Lines in Gatewaying 1496 When forwarding a message into or out of the Internet environment, a 1497 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1498 way a Received: line that is already in the header section. 1500 "Received:" header fields of messages originating from other 1501 environments may not conform exactly to this specification. However, 1502 the most important use of Received: lines is for debugging mail 1503 faults, and this debugging can be severely hampered by well-meaning 1504 gateways that try to "fix" a Received: line. As another consequence 1505 of trace header fields arising in non-SMTP environments, receiving 1506 systems MUST NOT reject mail based on the format of a trace header 1507 field and SHOULD be extremely robust in the light of unexpected 1508 information or formats in those header fields. 1510 The gateway SHOULD indicate the environment and protocol in the "via" 1511 clauses of Received header field(s) that it supplies. 1513 3.7.3. Addresses in Gatewaying 1515 From the Internet side, the gateway SHOULD accept all valid address 1516 formats in SMTP commands and in the RFC 822 header section, and all 1517 valid RFC 822 messages. Addresses and header fields generated by 1518 gateways MUST conform to applicable standards (including this one and 1519 RFC 5322 [12]). Gateways are, of course, subject to the same rules 1520 for handling source routes as those described for other SMTP systems 1521 in Section 3.3. 1523 3.7.4. Other Header Fields in Gatewaying 1525 The gateway MUST ensure that all header fields of a message that it 1526 forwards into the Internet mail environment meet the requirements for 1527 Internet mail. In particular, all addresses in "From:", "To:", 1528 "Cc:", etc., header fields MUST be transformed (if necessary) to 1529 satisfy the standard header syntax of RFC 5322 [12], MUST reference 1530 only fully-qualified domain names, and MUST be effective and useful 1531 for sending replies. The translation algorithm used to convert mail 1532 from the Internet protocols to another environment's protocol SHOULD 1533 ensure that error messages from the foreign mail environment are 1534 delivered to the reverse-path from the SMTP envelope, not to an 1535 address in the "From:", "Sender:", or similar header fields of the 1536 message. 1538 3.7.5. Envelopes in Gatewaying 1540 Similarly, when forwarding a message from another environment into 1541 the Internet, the gateway SHOULD set the envelope return path in 1542 accordance with an error message return address, if supplied by the 1543 foreign environment. If the foreign environment has no equivalent 1544 concept, the gateway must select and use a best approximation, with 1545 the message originator's address as the default of last resort. 1547 3.8. Terminating Sessions and Connections 1549 An SMTP connection is terminated when the client sends a QUIT 1550 command. The server responds with a positive reply code, after which 1551 it closes the connection. 1553 An SMTP server MUST NOT intentionally close the connection under 1554 normal operational circumstances (see Section 7.8) except: 1556 * After receiving a QUIT command and responding with a 221 reply. 1558 * After detecting the need to shut down the SMTP service and 1559 returning a 421 reply code. This reply code can be issued after 1560 the server receives any command or, if necessary, asynchronously 1561 from command receipt (on the assumption that the client will 1562 receive it after the next command is issued). 1564 * After a timeout, as specified in Section 4.5.3.2, occurs waiting 1565 for the client to send a command or data. 1567 In particular, a server that closes connections in response to 1568 commands that are not understood is in violation of this 1569 specification. Servers are expected to be tolerant of unknown 1570 commands, issuing a 500 reply and awaiting further instructions from 1571 the client. 1573 An SMTP server that is forcibly shut down via external means SHOULD 1574 attempt to send a line containing a 421 reply code to the SMTP client 1575 before exiting. The SMTP client will normally read the 421 reply 1576 code after sending its next command. 1578 SMTP clients that experience a connection close, reset, or other 1579 communications failure due to circumstances not under their control 1580 (in violation of the intent of this specification but sometimes 1581 unavoidable) SHOULD, to maintain the robustness of the mail system, 1582 treat the mail transaction as if a 421 response had been received and 1583 act accordingly. 1585 There are circumstances, contrary to the intent of this 1586 specification, in which an SMTP server may receive an indication that 1587 the underlying TCP connection has been closed or reset. To preserve 1588 the robustness of the mail system, SMTP servers SHOULD be prepared 1589 for this condition and SHOULD treat it as if a QUIT had been received 1590 before the connection disappeared. 1592 4. The SMTP Specifications 1594 4.1. SMTP Commands 1596 4.1.1. Command Semantics and Syntax 1598 The SMTP commands define the mail transfer or the mail system 1599 function requested by the user. SMTP commands are character strings 1600 terminated by . The commands themselves are alphabetic 1601 characters terminated by if parameters follow and 1602 otherwise. (In the interest of improved interoperability, SMTP 1603 receivers SHOULD tolerate trailing white space before the terminating 1604 .) The syntax of the local part of a mailbox MUST conform to 1605 receiver site conventions and the syntax specified in Section 4.1.2. 1606 The SMTP commands are discussed below. The SMTP replies are 1607 discussed in Section 4.2. 1609 A mail transaction involves several data objects that are 1610 communicated as arguments to different commands. The reverse-path is 1611 the argument of the MAIL command, the forward-path is the argument of 1612 the RCPT command, and the mail data is the argument of the DATA 1613 command. These arguments or data objects must be transmitted and 1614 held, pending the confirmation communicated by the end of mail data 1615 indication that finalizes the transaction. The model for this is 1616 that distinct buffers are provided to hold the types of data objects; 1617 that is, there is a reverse-path buffer, a forward-path buffer, and a 1618 mail data buffer. Specific commands cause information to be appended 1619 to a specific buffer, or cause one or more buffers to be cleared. 1621 Several commands (RSET, DATA, QUIT) are specified as not permitting 1622 parameters. In the absence of specific extensions offered by the 1623 server and accepted by the client, clients MUST NOT send such 1624 parameters and servers SHOULD reject commands containing them as 1625 having invalid syntax. 1627 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1629 These commands are used to identify the SMTP client to the SMTP 1630 server. The argument clause contains the fully-qualified domain name 1631 of the SMTP client, if one is available. In situations in which the 1632 SMTP client system does not have a meaningful domain name (e.g., when 1633 its address is dynamically allocated and no reverse mapping record is 1634 available), the client SHOULD send an address literal (see 1635 Section 4.1.3). Additional discussion of domain names in SMTP 1636 commands appears in Section 2.3.5. 1638 RFC 2821, and some earlier informal practices, encouraged following 1639 the literal by information that would help to identify the client 1640 system. That convention was not widely supported, and many SMTP 1641 servers considered it an error. In the interest of interoperability, 1642 it is probably wise for servers to be prepared for this string to 1643 occur, but SMTP clients SHOULD NOT send it. 1645 The SMTP server identifies itself to the SMTP client in the 1646 connection greeting reply and in the response to this command. 1648 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1649 command. If the SMTP server supports the SMTP service extensions, it 1650 will give a successful response, a failure response, or an error 1651 response. If the SMTP server, in violation of this specification, 1652 does not support any SMTP service extensions, it will generate an 1653 error response. Older client SMTP systems MAY, as discussed above, 1654 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1655 support the HELO command and reply properly to it. In any event, a 1656 client MUST issue HELO or EHLO before starting a mail transaction. 1658 These commands, and a "250 OK" reply to one of them, confirm that 1659 both the SMTP client and the SMTP server are in the initial state, 1660 that is, there is no transaction in progress and all state tables and 1661 buffers are cleared. 1663 Syntax: 1665 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1667 helo = "HELO" SP Domain CRLF 1668 Normally, the response to EHLO will be a multiline reply. Each line 1669 of the response contains a keyword and, optionally, one or more 1670 parameters. Following the normal syntax for multiline replies, these 1671 keywords follow the code (250) and a hyphen for all but the last 1672 line, and the code and a space for the last line. The syntax for a 1673 positive response, using the ABNF notation and terminal symbols of 1674 RFC 5234 [11], is: 1676 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1677 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1678 *( "250-" ehlo-line CRLF ) 1679 "250" SP ehlo-line CRLF ) 1681 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1682 ; string of any characters other than CR or LF 1684 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1686 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1687 ; additional syntax of ehlo-params depends on 1688 ; ehlo-keyword 1690 ehlo-param = 1*(%d33-126) 1691 ; any CHAR excluding and all 1692 ; control characters (US-ASCII 0-31 and 127 1693 ; inclusive) 1695 Although EHLO keywords may be specified in upper, lower, or mixed 1696 case, they MUST always be recognized and processed in a case- 1697 insensitive manner. This is simply an extension of practices 1698 specified in RFC 821 and Section 2.4. 1700 The EHLO response MUST contain keywords (and associated parameters if 1701 required) for all commands not listed as "required" in Section 4.5.1. 1703 4.1.1.2. MAIL (MAIL) 1705 This command is used to initiate a mail transaction in which the mail 1706 data is delivered to an SMTP server that may, in turn, deliver it to 1707 one or more mailboxes or pass it on to another system (possibly using 1708 SMTP). The argument clause contains a reverse-path and may contain 1709 optional parameters. In general, the MAIL command may be sent only 1710 when no mail transaction is in progress, see Section 4.1.4. 1712 The reverse-path consists of the sender mailbox. Historically, that 1713 mailbox might optionally have been preceded by a list of hosts, but 1714 that behavior is now deprecated (see Appendix F.2). In some types of 1715 reporting messages for which a reply is likely to cause a mail loop 1716 (for example, mail delivery and non-delivery notifications), the 1717 reverse-path may be null (see Section 3.6). 1719 This command clears the reverse-path buffer, the forward-path buffer, 1720 and the mail data buffer, and it inserts the reverse-path information 1721 from its argument clause into the reverse-path buffer. 1723 If service extensions were negotiated, the MAIL command may also 1724 carry parameters associated with a particular service extension. 1726 Syntax: 1728 mail = "MAIL FROM:" Reverse-path 1729 [SP Mail-parameters] CRLF 1731 4.1.1.3. RECIPIENT (RCPT) 1733 This command is used to identify an individual recipient of the mail 1734 data; multiple recipients are specified by multiple uses of this 1735 command. The argument clause contains a forward-path and may contain 1736 optional parameters. 1738 The forward-path consists of the required destination mailbox. When 1739 mail reaches its ultimate destination, the SMTP server inserts it 1740 into the destination mailbox in accordance with its host mail 1741 conventions. 1743 // JcK 20211128: above is new text in rfc5321bis-07, per notes from 1744 // Alexey and Ned, replacing the two paragraphs and text about source 1745 // routes that used to appear here. However, I'm a little concerned 1746 // about "ultimate destination" as used here. The earlier text was 1747 // about source routes and that term was defined as "the forward-path 1748 // contains only a destination mailbox)". But, without that context 1749 // and discussions about MDAs and what they might do, I am not sure I 1750 // know what the term means or if it is appropriate to talk about 1751 // SMTP servers inserting things in mailboxes if we can avoid it. 1753 // (JcK 20211214) Following significantly rewritten for rfc5321bis- 1754 // 08. 1756 Prior versions of the SMTP specification included text and examples 1757 in this section of use of the deprecated source route construct. If 1758 desired, see Appendix F.2 for discussion of that mechanism. 1760 This command appends its forward-path argument to the forward-path 1761 buffer; it does not change the reverse-path buffer nor the mail data 1762 buffer. 1764 For example, mail received at relay host xyz.com with envelope 1765 commands 1767 MAIL FROM: 1768 RCPT TO: 1770 will result in a DNS lookup for d.bar.org and transmission to the 1771 host specified in the most-preferred MX record that is available (or 1772 by the address record if there are no MX records). It will use 1773 envelope commands identical to the above, i.e., 1775 MAIL FROM: 1776 RCPT TO: 1778 Since hosts are not required to relay mail at all, xyz.com MAY also 1779 reject the message entirely when the RCPT command is received, using 1780 a 550 code (since this is a "policy reason"). 1782 If service extensions were negotiated, the RCPT command may also 1783 carry parameters associated with a particular service extension 1784 offered by the server. The client MUST NOT transmit parameters other 1785 than those associated with a service extension offered by the server 1786 in its EHLO response. 1788 Syntax: 1790 rcpt = "RCPT TO:" ( "" / "" / 1791 Forward-path ) [SP Rcpt-parameters] CRLF 1793 Note that, in a departure from the usual rules for 1794 local-parts, the "Postmaster" string shown above is 1795 treated as case-insensitive. 1797 4.1.1.4. DATA (DATA) 1799 The receiver normally sends a 354 response to DATA, and then treats 1800 the lines (strings ending in sequences, as described in 1801 Section 2.3.7) following the command as mail data from the sender. 1802 This command causes the mail data to be appended to the mail data 1803 buffer. The mail data may contain any of the 128 ASCII character 1804 codes, although experience has indicated that use of control 1805 characters other than SP, HT, CR, and LF may cause problems and 1806 SHOULD be avoided when possible. 1808 The mail data are terminated by a line containing only a period, that 1809 is, the character sequence ".", where the first is 1810 actually the terminator of the previous line (see Section 4.5.2). 1811 This is the end of mail data indication. The first of this 1812 terminating sequence is also the that ends the final line of 1813 the data (message text) or, if there was no mail data, ends the DATA 1814 command itself (the "no mail data" case does not conform to this 1815 specification since it would require that neither the trace header 1816 fields required by this specification nor the message header section 1817 required by RFC 5322 [12] be transmitted). An extra MUST NOT 1818 be added, as that would cause an empty line to be added to the 1819 message. The only exception to this rule would arise if the message 1820 body were passed to the originating SMTP-sender with a final "line" 1821 that did not end in ; in that case, the originating SMTP system 1822 MUST either reject the message as invalid or add in order to 1823 have the receiving SMTP server recognize the "end of data" condition. 1825 The custom of accepting lines ending only in , as a concession to 1826 non-conforming behavior on the part of some UNIX systems, has proven 1827 to cause more interoperability problems than it solves, and SMTP 1828 server systems MUST NOT do this, even in the name of improved 1829 robustness. In particular, the sequence "." (bare line 1830 feeds, without carriage returns) MUST NOT be treated as equivalent to 1831 . as the end of mail data indication. 1833 Receipt of the end of mail data indication requires the server to 1834 process the stored mail transaction information. This processing 1835 consumes the information in the reverse-path buffer, the forward-path 1836 buffer, and the mail data buffer, and on the completion of this 1837 command these buffers are cleared. If the processing is successful, 1838 the receiver MUST send an OK reply. If the processing fails, the 1839 receiver MUST send a failure reply. The SMTP model does not allow 1840 for partial failures at this point: either the message is accepted by 1841 the server for delivery and a positive response is returned or it is 1842 not accepted and a failure reply is returned. In sending a positive 1843 "250 OK" completion reply to the end of data indication, the receiver 1844 takes full responsibility for the message (see Section 6.1). Errors 1845 that are diagnosed subsequently MUST be reported in a mail message, 1846 as discussed in Section 4.4. 1848 When the SMTP server accepts a message either for relaying or for 1849 final delivery, it inserts a trace record (also referred to 1850 interchangeably as a "time stamp line" or "Received" line) at the top 1851 of the mail data. This trace record indicates the identity of the 1852 host that sent the message, the identity of the host that received 1853 the message (and is inserting this time stamp), and the date and time 1854 the message was received. Relayed messages will have multiple time 1855 stamp lines. Details for formation of these lines, including their 1856 syntax, is specified in Section 4.4. 1858 Additional discussion about the operation of the DATA command appears 1859 in Section 3.3. 1861 Syntax: 1863 data = "DATA" CRLF 1865 4.1.1.5. RESET (RSET) 1867 This command specifies that the current mail transaction will be 1868 aborted. Any stored sender, recipients, and mail data MUST be 1869 discarded, and all buffers and state tables cleared. The receiver 1870 MUST send a "250 OK" reply to a RSET command with no arguments. A 1871 reset command may be issued by the client at any time. It is 1872 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1873 immediately after EHLO, before EHLO is issued in the session, after 1874 an end of data indicator has been sent and acknowledged, or 1875 immediately before a QUIT. An SMTP server MUST NOT close the 1876 connection as the result of receiving a RSET; that action is reserved 1877 for QUIT (see Section 4.1.1.10). 1879 Since EHLO implies some additional processing and response by the 1880 server, RSET will normally be more efficient than reissuing that 1881 command, even though the formal semantics are the same. 1883 Syntax: 1885 rset = "RSET" CRLF 1887 4.1.1.6. VERIFY (VRFY) 1889 This command asks the receiver to confirm that the argument 1890 identifies a user or mailbox. If it is a user name, information is 1891 returned as specified in Section 3.5. 1893 This command has no effect on the reverse-path buffer, the forward- 1894 path buffer, or the mail data buffer. 1896 Syntax: 1898 vrfy = "VRFY" SP String CRLF 1900 4.1.1.7. EXPAND (EXPN) 1902 This command asks the receiver to confirm that the argument 1903 identifies a mailing list, and if so, to return the membership of 1904 that list. If the command is successful, a reply is returned 1905 containing information as described in Section 3.5. This reply will 1906 have multiple lines except in the trivial case of a one-member list. 1908 This command has no effect on the reverse-path buffer, the forward- 1909 path buffer, or the mail data buffer, and it may be issued at any 1910 time. 1912 Syntax: 1914 expn = "EXPN" SP String CRLF 1916 4.1.1.8. HELP (HELP) 1918 This command causes the server to send helpful information to the 1919 client. The command MAY take an argument (e.g., any command name) 1920 and return more specific information as a response. 1922 This command has no effect on the reverse-path buffer, the forward- 1923 path buffer, or the mail data buffer, and it may be issued at any 1924 time. 1926 SMTP servers SHOULD support HELP without arguments and MAY support it 1927 with arguments. 1929 Syntax: 1931 help = "HELP" [ SP String ] CRLF 1933 4.1.1.9. NOOP (NOOP) 1935 This command does not affect any parameters or previously entered 1936 commands. It specifies no action other than that the receiver send a 1937 "250 OK" reply. 1939 This command has no effect on the reverse-path buffer, the forward- 1940 path buffer, or the mail data buffer, and it may be issued at any 1941 time. If a parameter string is specified, servers SHOULD ignore it. 1943 Syntax: 1945 noop = "NOOP" [ SP String ] CRLF 1947 4.1.1.10. QUIT (QUIT) 1949 This command specifies that the receiver MUST send a "221 OK" reply, 1950 and then close the transmission channel. 1952 The receiver MUST NOT intentionally close the transmission channel 1953 until it receives and replies to a QUIT command (even if there was an 1954 error). The sender MUST NOT intentionally close the transmission 1955 channel until it sends a QUIT command, and it SHOULD wait until it 1956 receives the reply (even if there was an error response to a previous 1957 command). If the connection is closed prematurely due to violations 1958 of the above or system or network failure, the server MUST cancel any 1959 pending transaction, but not undo any previously completed 1960 transaction, and generally MUST act as if the command or transaction 1961 in progress had received a temporary error (i.e., a 4yz response). 1963 The QUIT command may be issued at any time. Any current uncompleted 1964 mail transaction will be aborted. 1966 Syntax: 1968 quit = "QUIT" CRLF 1970 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1972 If the server SMTP does not recognize or cannot implement one or more 1973 of the parameters associated with a particular MAIL or RCPT command, 1974 it will return code 555. 1976 If, for some reason, the server is temporarily unable to accommodate 1977 one or more of the parameters associated with a MAIL or RCPT command, 1978 and if the definition of the specific parameter does not mandate the 1979 use of another code, it should return code 455. 1981 Errors specific to particular parameters and their values will be 1982 specified in the parameter's defining RFC. 1984 4.1.2. Command Argument Syntax 1986 The syntax of the argument clauses of the above commands (using the 1987 syntax specified in RFC 5234 [11] where applicable) is given below. 1988 Some terminals not defined in this document, but are defined 1989 elsewhere, specifically: 1991 * In the "core" syntax in Appendix B of RFC 5234 [11]: ALPHA , CRLF 1992 , DIGIT , HEXDIG , and SP 1994 * In the message format syntax in RFC 5322 [12]: atext , CFWS , and 1995 FWS . 1997 Reverse-path = Path / "<>" 1999 Forward-path = Path 2001 Path = "<" Mailbox ">" 2003 Mail-parameters = esmtp-param *(SP esmtp-param) 2005 Rcpt-parameters = esmtp-param *(SP esmtp-param) 2007 esmtp-param = esmtp-keyword ["=" esmtp-value] 2009 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 2011 esmtp-value = 1*(%d33-60 / %d62-126) 2012 ; any CHAR excluding "=", SP, and control 2013 ; characters. If this string is an email address, 2014 ; i.e., a Mailbox, then the "xtext" syntax [34] 2015 ; SHOULD be used. 2017 Keyword = Ldh-str 2019 Argument = Atom 2021 Domain = sub-domain *("." sub-domain) 2023 sub-domain = Let-dig [Ldh-str] 2025 Let-dig = ALPHA / DIGIT 2027 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2029 address-literal = "[" ( IPv4-address-literal / 2030 IPv6-address-literal / 2031 General-address-literal ) "]" 2032 ; See Section 4.1.3 2034 Mailbox = Local-part "@" ( Domain / address-literal ) 2036 Local-part = Dot-string / Quoted-string 2037 ; MAY be case-sensitive 2039 Dot-string = Atom *("." Atom) 2041 Atom = 1*atext 2043 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2045 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2047 quoted-pairSMTP = %d92 %d32-126 2048 ; i.e., backslash followed by any ASCII 2049 ; graphic (including itself) or SPace 2051 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2052 ; i.e., within a quoted string, any 2053 ; ASCII graphic or space is permitted 2054 ; without backslash-quoting except 2055 ; double-quote and the backslash itself. 2057 String = Atom / Quoted-string 2059 // JcK 20211128: Following two paragraphs reordered and text added to 2060 // the (now) second one with statements about equivalence and 2061 // examples. I proposed to drop "semantically" entirely from the 2062 // description if there are no objections. 2063 Note that the backslash, "\", is a quote character, which is used to 2064 indicate that the next character is to be used literally (instead of 2065 its normal interpretation). For example, "Joe\,Smith" indicates a 2066 single nine-character user name string with the comma being the 2067 fourth character of that string. 2069 While the above definition for Local-part is relatively permissive, 2070 for maximum interoperability, a mailbox SHOULD NOT be defined with 2071 Local-part requiring (or using) the Quoted-string form or with the 2072 Local-part being case-sensitive. Further, when comparing a Local- 2073 part (e.g., to a specific mailbox name), all quoting MUST be treated 2074 as equivalent. A sending system SHOULD transmit the form that uses 2075 the minimum quoting possible. 2077 For example, the following 3 local-parts are equivalent and MUST 2078 compare equal: "ab cd ef", "ab\ cd ef" and "ab\ \cd ef". 2079 Similarly, "fred" and fred must compare equal. White space 2080 reduction MUST NOT be applied to Local-part by intermediate 2081 systems. 2083 Systems MUST NOT define mailboxes in such a way as to require the use 2084 in SMTP of non-ASCII characters (octets with the high order bit set 2085 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2086 These characters MUST NOT be used in MAIL or RCPT commands or other 2087 commands that require mailbox names. 2089 To promote interoperability and consistent with long-standing 2090 guidance about conservative use of the DNS in naming and applications 2091 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2092 characters outside the set of alphabetic characters, digits, and 2093 hyphen MUST NOT appear in domain name labels for SMTP clients or 2094 servers. In particular, the underscore character is not permitted. 2095 SMTP servers that receive a command in which invalid character codes 2096 have been employed, and for which there are no other reasons for 2097 rejection, MUST reject that command with a 501 response (this rule, 2098 like others, could be overridden by appropriate SMTP extensions). 2100 4.1.3. Address Literals 2102 Sometimes a host is not known to the domain name system and 2103 communication (and, in particular, communication to report and repair 2104 the error) is blocked. To bypass this barrier, a special literal 2105 form of the address is allowed as an alternative to a domain name. 2106 For IPv4 addresses, this form uses four small decimal integers 2107 separated by dots and enclosed by brackets such as [123.255.37.2], 2108 which indicates an (IPv4) Internet Address in sequence-of-octets 2109 form. For IPv6 and other forms of addressing that might eventually 2110 be standardized, the form consists of a standardized "tag" that 2111 identifies the address syntax, a colon, and the address itself, in a 2112 format specified as part of the relevant standards (i.e., RFC 4291 2113 [10] for IPv6). 2115 // [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet another 2116 // modification to the IPv6 address literal syntax, based on part on 2117 // RFC 5952. We should consider whether those, or other, 2118 // modifications are appropriate and/or whether, given both the 2119 // issues of spam/malware and servers supporting multiple domains, it 2120 // it time to deprecate mailboxes containing address literals 2121 // entirely (EHLO fields may be a different issue). If we are going 2122 // to allow IPv6 address literals, it may be time to incorporate 2123 // something by reference rather than including specific syntax here 2124 // (RFC 5952 is 14 pages long and does not contain any ABNF). 2126 Specifically: 2128 IPv4-address-literal = Snum 3("." Snum) 2130 IPv6-address-literal = "IPv6:" IPv6-addr 2132 General-address-literal = Standardized-tag ":" 1*dcontent 2134 Standardized-tag = Ldh-str 2135 ; Standardized-tag MUST be specified in a 2136 ; Standards-Track RFC and registered with IANA 2138 dcontent = %d33-90 / ; Printable US-ASCII 2139 %d94-126 ; excl. "[", "\", "]" 2141 Snum = 1*3DIGIT 2142 ; representing a decimal integer 2143 ; value in the range 0 through 255 2145 IPv6-addr = 6( h16 ":" ) ls32 2146 / "::" 5( h16 ":" ) ls32 2147 / [ h16 ] "::" 4( h16 ":" ) ls32 2148 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2149 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2150 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2151 / [ *4( h16 ":" ) h16 ] "::" ls32 2152 / [ *5( h16 ":" ) h16 ] "::" h16 2153 / [ *6( h16 ":" ) h16 ] "::" 2154 ; This definition is consistent with the one for 2155 ; URIs [40]. 2157 ls32 = ( h16 ":" h16 ) / IPv4address 2158 ; least-significant 32 bits of address 2160 h16 = 1*4HEXDIG 2161 ; 16 bits of address represented in hexadecimal 2163 4.1.4. Order of Commands 2165 There are restrictions on the order in which these commands may be 2166 used. 2168 A session that will contain mail transactions MUST first be 2169 initialized by the use of the EHLO command. An SMTP server SHOULD 2170 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2171 without this initialization. 2173 An EHLO command MAY be issued by a client later in the session. If 2174 it is issued after the session begins and the EHLO command is 2175 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2176 and reset the state exactly as if a RSET command had been issued 2177 (specifically, it terminates any mail transaction that was in 2178 progress, see Section 3.3). In other words, the sequence of RSET 2179 followed immediately by EHLO is redundant, but not harmful other than 2180 in the performance cost of executing unnecessary commands. However 2181 the response to an additional EHLO command MAY be different from that 2182 from prior ones; the client MUST rely only on the responses from the 2183 most recent EHLO command. 2185 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2186 502, or 550 failure replies MUST be returned as appropriate. The 2187 SMTP server MUST stay in the same state after transmitting these 2188 replies that it was in before the EHLO was received. 2190 The SMTP client MUST, if possible, ensure that the domain parameter 2191 to the EHLO command is a primary host name as specified for this 2192 command in Section 2.3.5. If this is not possible (e.g., when the 2193 client's address is dynamically assigned and the client does not have 2194 an obvious name), an address literal SHOULD be substituted for the 2195 domain name. 2197 An SMTP server MAY verify that the domain name argument in the EHLO 2198 command has an address record matching the IP address of the client. 2200 // JcK 20211022: Note that Alessandro's email of 2021-10-13 proposes 2201 // adding "See [A/S] for further discussion." after that sentence. 2202 // Noting that phrasing could get us in trouble if the A/S takes a 2203 // long time to complete, can the WG please make a decision? 2205 // JcK 20211022: Additional question: should we be clear that this 2206 // refers to a forward lookup of the domain name, not a reverse 2207 // lookup of the address? 2208 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2209 during a session, or without previously initializing a session. SMTP 2210 servers SHOULD process these normally (that is, not return a 503 2211 code) even if no EHLO command has yet been received; clients SHOULD 2212 open a session with EHLO before sending these commands. 2214 If these rules are followed, the example in RFC 821 that shows "550 2215 access denied to you" in response to an EXPN command is incorrect 2216 unless an EHLO command precedes the EXPN or the denial of access is 2217 based on the client's IP address or other authentication or 2218 authorization-determining mechanisms. 2220 A mail transaction begins with a MAIL command and then consists of 2221 one or more RCPT commands, and a DATA command, in that order. A mail 2222 transaction may be aborted by the RSET, a new EHLO, or the QUIT 2223 command. 2225 SMTP extensions (see Section 2.2) may create additional commands that 2226 initiate, abort, or end the transaction.More generally, any new 2227 command MUST clearly document any effect it has on the transaction 2228 state. 2230 There may be zero or more transactions in a session. MAIL MUST NOT 2231 be sent if a mail transaction is already open, i.e., it should be 2232 sent only if no mail transaction had been started in the session, or 2233 if the previous one successfully concluded with a successful DATA 2234 command, or if the previous one was aborted, e.g., with a RSET or new 2235 EHLO. 2236 // [5321bis] See comment about changing this convoluted discussion to 2237 // talk about 'mail transaction' above. --Jck (and see Ticket #11 2238 // correspondence with Alexey 2021-07-06) 2240 If the transaction beginning command argument is not acceptable, a 2241 501 failure reply MUST be returned and the SMTP server MUST stay in 2242 the same state. If the commands in a transaction are out of order to 2243 the degree that they cannot be processed by the server, a 503 failure 2244 reply MUST be returned and the SMTP server MUST stay in the same 2245 state. 2247 The last command in a session MUST be the QUIT command. The QUIT 2248 command SHOULD be used by the client SMTP to request connection 2249 closure, even when no session opening command was sent and accepted. 2251 4.2. SMTP Replies 2253 Replies to SMTP commands serve to ensure the synchronization of 2254 requests and actions in the process of mail transfer and to guarantee 2255 that the SMTP client always knows the state of the SMTP server. 2256 Every command MUST generate exactly one reply. 2258 The details of the command-reply sequence are described in 2259 Section 4.3. 2261 An SMTP reply consists of a three digit number (transmitted as three 2262 numeric characters) followed by some text unless specified otherwise 2263 in this document. The number is for use by automata to determine 2264 what state to enter next; the text is for the human user. The three 2265 digits contain enough encoded information that the SMTP client need 2266 not examine the text and may either discard it or pass it on to the 2267 user, as appropriate. Exceptions are as noted elsewhere in this 2268 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2269 are associated with message text that must be parsed and interpreted 2270 by machines. In the general case, the text may be receiver dependent 2271 and context dependent, so there are likely to be varying texts for 2272 each reply code. A discussion of the theory of reply codes is given 2273 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2274 three-digit code, , one line of text, and , or a multiline 2275 reply (as defined in the same section). Since, in violation of this 2276 specification, the text is sometimes not sent, clients that do not 2277 receive it SHOULD be prepared to process the code alone (with or 2278 without a trailing space character). Only the EHLO, EXPN, and HELP 2279 commands are expected to result in multiline replies in normal 2280 circumstances; however, multiline replies are allowed for any 2281 command. 2283 In ABNF, server responses are: 2285 Greeting = ( "220 " (Domain / address-literal) 2286 [ SP textstring ] CRLF ) / 2287 ( "220-" (Domain / address-literal) 2288 [ SP textstring ] CRLF 2289 *( "220-" [ textstring ] CRLF ) 2290 "220" [ SP textstring ] CRLF ) 2292 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2294 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2295 Reply-code [ SP textstring ] CRLF 2297 Reply-code = %x32-35 %x30-35 %x30-39 2298 where "Greeting" appears only in the 220 response that announces that 2299 the server is opening its part of the connection. (Other possible 2300 server responses upon connection follow the syntax of Reply-line.) 2302 An SMTP server SHOULD send only the reply codes listed in this 2303 document or additions to the list as discussed below. An SMTP server 2304 SHOULD use the text shown in the examples whenever appropriate. 2306 An SMTP client MUST determine its actions only by the reply code, not 2307 by the text (except for the "change of address" 251 and 551 and, if 2308 necessary, 220, 221, and 421 replies); in the general case, any text, 2309 including no text at all (although senders SHOULD NOT send bare 2310 codes), MUST be acceptable. The space (blank) following the reply 2311 code is considered part of the text. A Sender-SMTP MUST first test 2312 the whole 3 digit reply code it receives, as well as any accompanying 2313 supplemental codes or information (see RFC 3463 [7] and RFC 5248 2314 [43]). If the full reply code is not recognized, and the additional 2315 information is not recognized or missing, the Sender-SMTP MUST use 2316 the first digit (severity indication) of a reply code it receives. 2318 The list of codes that appears below MUST NOT be construed as 2319 permanent. While the addition of new codes should be a rare and 2320 significant activity, with supplemental information in the textual 2321 part of the response (including enhanced status codes [7] and the 2322 successors to that specification) being preferred, new codes may be 2323 added as the result of new Standards or Standards-Track 2324 specifications. Consequently, a sender-SMTP MUST be prepared to 2325 handle codes not specified in this document and MUST do so by 2326 interpreting the first digit only. 2328 In the absence of extensions negotiated with the client, SMTP servers 2329 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2330 or 5. Clients that receive such out-of-range codes SHOULD normally 2331 treat them as fatal errors and terminate the mail transaction. 2333 4.2.1. Reply Code Severities and Theory 2335 The three digits of the reply each have a special significance. The 2336 first digit denotes whether the response is good, bad, or incomplete. 2337 An unsophisticated SMTP client, or one that receives an unexpected 2338 code, will be able to determine its next action (proceed as planned, 2339 redo, retrench, etc.) by examining this first digit. An SMTP client 2340 that wants to know approximately what kind of error occurred (e.g., 2341 mail system error, command syntax error) may examine the second 2342 digit. The third digit and any supplemental information that may be 2343 present is reserved for the finest gradation of information. 2345 There are four values for the first digit of the reply code: 2347 2yz Positive Completion reply 2348 The requested action has been successfully completed. A new 2349 request may be initiated. 2351 3yz Positive Intermediate reply 2352 The command has been accepted, but the requested action is being 2353 held in abeyance, pending receipt of further information. The 2354 SMTP client should send another command specifying this 2355 information. This reply is used in command sequence groups (i.e., 2356 in DATA). 2358 4yz Transient Negative Completion reply 2359 The command was not accepted, and the requested action did not 2360 occur. However, the error condition is temporary, and the action 2361 may be requested again. The sender should return to the beginning 2362 of the command sequence (if any). It is difficult to assign a 2363 meaning to "transient" when two different sites (receiver- and 2364 sender-SMTP agents) must agree on the interpretation. Each reply 2365 in this category might have a different time value, but the SMTP 2366 client SHOULD try again. A rule of thumb to determine whether a 2367 reply fits into the 4yz or the 5yz category (see below) is that 2368 replies are 4yz if they can be successful if repeated without any 2369 change in command form or in properties of the sender or receiver 2370 (that is, the command is repeated identically and the receiver 2371 does not put up a new implementation). 2373 5yz Permanent Negative Completion reply 2374 The command was not accepted and the requested action did not 2375 occur. The SMTP client SHOULD NOT repeat the exact request (in 2376 the same sequence). Even some "permanent" error conditions can be 2377 corrected, so the human user may want to direct the SMTP client to 2378 reinitiate the command sequence by direct action at some point in 2379 the future (e.g., after the spelling has been changed, or the user 2380 has altered the account status). 2382 It is worth noting that the file transfer protocol (FTP) [15] uses a 2383 very similar code architecture and that the SMTP codes are based on 2384 the FTP model. However, SMTP uses a one-command, one-response model 2385 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2386 SMTP model. 2388 The second digit encodes responses in specific categories: 2390 x0z Syntax: These replies refer to syntax errors, syntactically 2391 correct commands that do not fit any functional category, and 2392 unimplemented or superfluous commands. 2394 x1z Information: These are replies to requests for information, such 2395 as status or help. 2397 x2z Connections: These are replies referring to the transmission 2398 channel. 2400 x3z Unspecified. 2402 x4z Unspecified. 2404 x5z Mail system: These replies indicate the status of the receiver 2405 mail system vis-a-vis the requested transfer or other mail system 2406 action. 2408 The third digit gives a finer gradation of meaning in each category 2409 specified by the second digit. The list of replies illustrates this. 2410 Each reply text is recommended rather than mandatory, and may even 2411 change according to the command with which it is associated. On the 2412 other hand, the reply codes must strictly follow the specifications 2413 in this section. Receiver implementations should not invent new 2414 codes for slightly different situations from the ones described here, 2415 but rather adapt codes already defined. 2417 For example, a command such as NOOP, whose successful execution does 2418 not offer the SMTP client any new information, will return a 250 2419 reply. The reply is 502 when the command requests an unimplemented 2420 non-site-specific action. A refinement of that is the 504 reply for 2421 a command that is implemented, but that requests an unimplemented 2422 parameter. 2424 The reply text may be longer than a single line; in these cases the 2425 complete text must be marked so the SMTP client knows when it can 2426 stop reading the reply. This requires a special format to indicate a 2427 multiple line reply. 2429 The format for multiline replies requires that every line, except the 2430 last, begin with the reply code, followed immediately by a hyphen, 2431 "-" (also known as minus), followed by text. The last line will 2432 begin with the reply code, followed immediately by , optionally 2433 some text, and . As noted above, servers SHOULD send the 2434 if subsequent text is not sent, but clients MUST be prepared for it 2435 to be omitted. 2437 For example: 2439 250-First line 2440 250-Second line 2441 250-234 Text beginning with numbers 2442 250 The last line 2444 In a multiline reply, the reply code on each of the lines MUST be the 2445 same. It is reasonable for the client to rely on this, so it can 2446 make processing decisions based on the code in any line, assuming 2447 that all others will be the same. In a few cases, there is important 2448 data for the client in the reply "text". The client will be able to 2449 identify these cases from the current context. 2451 4.2.2. Reply Codes by Function Groups 2453 500 Syntax error, command unrecognized (This may include errors such 2454 as command line too long) 2456 501 Syntax error in parameters or arguments 2458 502 Command not implemented (see Section 4.2.4.1) 2460 503 Bad sequence of commands 2462 504 Command parameter not implemented 2464 211 System status, or system help reply 2466 214 Help message (Information on how to use the receiver or the 2467 meaning of a particular non-standard command; this reply is useful 2468 only to the human user) 2470 220 Service ready 2472 221 Service closing transmission channel 2474 421 Service not available, closing transmission channel 2475 (This may be a reply to any command if the service knows it must 2476 shut down) 2478 521 No mail service here. 2480 556 No mail service at this domain. 2482 250 Requested mail action okay, completed 2484 251 User not local; will forward to (See 2485 Section 3.4.1) 2487 252 Cannot VRFY user, but will accept message and attempt delivery 2488 (See Section 3.5.3) 2490 455 Server unable to accommodate parameters 2491 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2493 450 Requested mail action not taken: mailbox unavailable (e.g., 2494 mailbox busy or temporarily blocked for policy reasons) 2496 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2497 not found, no access, or command rejected for policy reasons) 2499 451 Requested action aborted: error in processing 2501 551 User not local; please try (See Section 3.4.1) 2503 452 Requested action not taken: insufficient system storage 2504 (preferred code for "too many recipients", see Section 4.5.3.1.10) 2506 552 Requested mail action aborted: exceeded storage allocation. 2508 553 Requested action not taken: mailbox name not allowed (e.g., 2509 mailbox syntax incorrect) 2511 354 Start mail input; end with . 2513 554 Transaction failed (Or, historically in the case of a 2514 connection-opening response, "No SMTP service here". 521 is now 2515 preferred for that function at connection-opening if the server 2516 never accepts mail.) 2518 // [5321bis] [[Note in Draft: Revise above statement in the light 2519 of 2520 // new 521 code?? -- revised with rfc5321bis-04]] 2522 4.2.3. Reply Codes in Numeric Order 2524 211 System status, or system help reply 2526 214 Help message (Information on how to use the receiver or the 2527 meaning of a particular non-standard command; this reply is useful 2528 only to the human user) 2530 220 Service ready 2532 221 Service closing transmission channel 2534 250 Requested mail action okay, completed 2536 251 User not local; will forward to (See 2537 Section 3.4.1) 2539 252 Cannot VRFY user, but will accept message and attempt delivery 2540 (See Section 3.5.3) 2542 354 Start mail input; end with . 2544 421 Service not available, closing transmission channel 2545 (This may be a reply to any command if the service knows it must 2546 shut down) 2548 450 Requested mail action not taken: mailbox unavailable (e.g., 2549 mailbox busy or temporarily blocked for policy reasons) 2551 451 Requested action aborted: local error in processing 2553 452 Requested action not taken: insufficient system storage (also 2554 preferred code for "too many recipients", see Section 4.5.3.1.10) 2556 455 Server unable to accommodate parameters 2558 500 Syntax error, command unrecognized (This may include errors such 2559 as command line too long) 2561 501 Syntax error in parameters or arguments 2563 502 Command not implemented (see Section 4.2.4.1) 2565 503 Bad sequence of commands 2567 504 Command parameter not implemented 2569 521 No mail service (See Section 4.2.4.2.) 2571 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2572 not found, no access, or command rejected for policy reasons) 2574 551 User not local; please try (See Section 3.4.1) 2576 552 Requested mail action aborted: exceeded storage allocation. 2578 553 Requested action not taken: mailbox name not allowed (e.g., 2579 mailbox syntax incorrect) 2581 554 Transaction failed (Or, in the case of a connection-opening 2582 response, "No SMTP service here" although 521 is now preferred for 2583 the latter. See Section 4.2.4.2.) 2585 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2586 556 No mail service at this domain. (See Section 4.2.4.2.) 2588 4.2.4. Some specific code situations and relationships 2590 4.2.4.1. Reply Code 502 2592 Questions have been raised as to when reply code 502 (Command not 2593 implemented) SHOULD be returned in preference to other codes. 502 2594 SHOULD be used when the command is actually recognized by the SMTP 2595 server, but not implemented. If the command is not recognized, code 2596 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2597 capabilities in response to EHLO for which they will return 502 (or 2598 500) replies. 2600 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2602 Codes 521, 554, and 556 are all used to report different types of "no 2603 mail accepted" situations. They differ as follows. 521 is an 2604 indication from a system answering on the SMTP port that it does not 2605 support SMTP service (a so-called "dummy server" as discussed in RFC 2606 7504 [45] and elsewhere). Obviously, it requires that system exist 2607 and that a connection can be made successfully to it. Because a 2608 system that does not accept any mail cannot meaningfully accept a 2609 RCPT command, any commands (other than QUIT) issued after an SMTP 2610 server has issued a 521 reply are client (sender) errors. 2612 When a domain does not intend to accept mail and wishes to publish 2613 that fact rather than being subjected to connection attempts, the 2614 best way to accomplish that is to use the "Null MX" convention. This 2615 is done by advertising a single MX RR (see Section 3.3.9 of (RFC 1035 2616 [4]) with an RDATA section consisting of preference number 0 and a 2617 zero-length label, written in master files as ".", as the exchange 2618 domain, to denote that there exists no mail exchanger for that 2619 domain. Reply code 556 is then used by a message submission or 2620 intermediate SMTP system (see Section 1.1) to report that it cannot 2621 forward the message further because it knows from the DNS entry that 2622 the recipient domain does not accept mail. If, despite publishing 2623 the DNS entry, the server domain chooses to respond on the SMTP port, 2624 it SHOULD respond with the 556 code as well. The details of the Null 2625 MX convention were first defined in RFC 7505 [46]; see that document 2626 for additional discussion of the rationale for that convention. 2628 Reply code 554 would normally be used in response to a RCPT command 2629 (or extension command with similar intent) when the SMTP system 2630 identifies a domain that it can (or has) determined never accepts 2631 mail. Other codes, including 554 and the temporary 450, are used for 2632 more transient situations and situations in which an SMTP server 2633 cannot or will not deliver to (or accept mail for) a particular 2634 system or mailbox for policy reasons rather than ones directly 2635 related to SMTP processing. 2637 // [JcK 20210904]: do we want/need to discuss temporary server 2638 // outages? And is the discussion above sufficient to obsolete RFC 2639 // 7505 or do we need either more text or some pretense to claim to 2640 // update it. 2642 4.2.4.3. Reply Codes after DATA and the Subsequent . 2644 When an SMTP server returns a positive completion status (2yz code) 2645 after the DATA command is completed with ., it accepts 2646 responsibility for: 2648 * delivering the message (if the recipient mailbox exists), or 2650 * if attempts to deliver the message fail due to transient 2651 conditions, retrying delivery some reasonable number of times at 2652 intervals as specified in Section 4.5.4. 2654 * if attempts to deliver the message fail due to permanent 2655 conditions, or if repeated attempts to deliver the message fail 2656 due to transient conditions, returning appropriate notification to 2657 the sender of the original message (using the address in the SMTP 2658 MAIL command). 2660 When an SMTP server returns a temporary error status (4yz) code after 2661 the DATA command is completed with ., it MUST NOT make a 2662 subsequent attempt to deliver that message. The SMTP client retains 2663 responsibility for the delivery of that message and may either return 2664 it to the user or requeue it for a subsequent attempt (see 2665 Section 4.5.4.1). 2667 The user who originated the message SHOULD be able to interpret the 2668 return of a transient failure status (by mail message or otherwise) 2669 as a non-delivery indication, just as a permanent failure would be 2670 interpreted. If the client SMTP successfully handles these 2671 conditions, the user will not receive such a reply. 2673 When an SMTP server returns a permanent error status (5yz) code after 2674 the DATA command is completed with ., it MUST NOT make 2675 any subsequent attempt to deliver the message. As with temporary 2676 error status codes, the SMTP client retains responsibility for the 2677 message, but SHOULD NOT again attempt delivery to the same server 2678 without user review of the message and response and appropriate 2679 intervention. 2681 4.3. Sequencing of Commands and Replies 2683 4.3.1. Sequencing Overview 2685 The communication between the sender and receiver is an alternating 2686 dialogue, controlled by the sender. As such, the sender issues a 2687 command and the receiver responds with a reply. Unless other 2688 arrangements are negotiated through service extensions, the sender 2689 MUST wait for this response before sending further commands. One 2690 important reply is the connection greeting. Normally, a receiver 2691 will send a 220 "Service ready" reply when the connection is 2692 completed. The sender SHOULD wait for this greeting message before 2693 sending any commands. 2695 Note: all the greeting-type replies have the official name (the 2696 fully-qualified primary domain name) of the server host as the first 2697 word following the reply code. Sometimes the host will have no 2698 meaningful name. See Section 4.1.3 for a discussion of alternatives 2699 in these situations. 2701 For example, 2703 220 ISIF.USC.EDU Service ready 2705 or 2707 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2709 or 2711 220 [10.0.0.1] Clueless host service ready 2713 The table below lists alternative success and failure replies for 2714 each command. These SHOULD be strictly adhered to. A receiver MAY 2715 substitute text in the replies, but the meanings and actions implied 2716 by the code numbers and by the specific command reply sequence MUST 2717 be preserved. However, in order to provide robustness as SMTP is 2718 extended and evolves, the discussion in Section 4.2.1 still applies: 2719 all SMTP clients MUST be prepared to accept any code that conforms to 2720 the discussion in that section and MUST be prepared to interpret it 2721 on the basis of its first digit only. 2723 4.3.2. Command-Reply Sequences 2725 Each command is listed with its usual possible replies. The prefixes 2726 used before the possible replies are "I" for intermediate, "S" for 2727 success, and "E" for error. Since some servers may generate other 2728 replies under special circumstances, and to allow for future 2729 extension, SMTP clients SHOULD, when possible, interpret only the 2730 first digit of the reply and MUST be prepared to deal with 2731 unrecognized reply codes by interpreting the first digit only. 2732 Unless extended using the mechanisms described in Section 2.2, SMTP 2733 servers MUST NOT transmit reply codes to an SMTP client that are 2734 other than three digits or that do not start in a digit between 2 and 2735 5 inclusive. 2737 These sequencing rules and, in principle, the codes themselves, can 2738 be extended or modified by SMTP extensions offered by the server and 2739 accepted (requested) by the client. However, if the target is more 2740 precise granularity in the codes, rather than codes for completely 2741 new purposes, the system described in RFC 3463 [7] SHOULD be used in 2742 preference to the invention of new codes. 2744 In addition to the codes listed below, any SMTP command can return 2745 any of the following codes if the corresponding unusual circumstances 2746 are encountered: 2748 500 For the "command line too long" case or if the command name was 2749 not recognized. Note that producing a "command not recognized" 2750 error in response to the required subset of these commands is a 2751 violation of this specification. Similarly, producing a "command 2752 too long" message for a command line shorter than 512 characters 2753 would violate the provisions of Section 4.5.3.1.4. 2755 501 Syntax error in command or arguments. In order to provide for 2756 future extensions, commands that are specified in this document as 2757 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2758 message if arguments are supplied in the absence of EHLO- 2759 advertised extensions. 2761 421 Service shutting down and closing transmission channel 2763 Specific sequences are: 2765 CONNECTION ESTABLISHMENT 2767 - S: 220 2768 E: 521, 554, 556 2770 EHLO or HELO 2772 - S: 250 2773 E: 504 (a conforming implementation could return this code only 2774 in fairly obscure cases), 550, 502 (permitted only with an old- 2775 style server that does not support EHLO) 2777 MAIL 2779 - S: 250 2780 E: 552, 451, 452, 550, 553, 503, 455, 555 2782 RCPT 2784 - S: 250, 251 (but see Section 3.4.1 for discussion of 251 and 2785 551) 2786 E: 550, 551, 552 (obsolete for "too many recipients; see 2787 Section 4.5.3.1.10, 553, 450, 451, 452, 503, 455, 555 2789 DATA 2791 - I: 354 -> data -> S: 250 2793 o E: 552, 554, 451, 452 2795 o E: 450, 550 (rejections for policy reasons) 2797 - E: 503, 554 2799 RSET 2801 - S: 250 2803 VRFY 2805 - S: 250, 251, 252 2806 E: 550, 551, 553, 502, 504 2808 EXPN 2809 - S: 250, 252 2810 E: 550, 500, 502, 504 2812 HELP 2814 - S: 211, 214 2815 E: 502, 504 2817 NOOP 2819 - S: 250 2821 QUIT 2823 - S: 221 2825 4.4. Trace Information 2827 Trace information is used to provide an audit trail of message 2828 handling. In addition, it indicates a route back to the sender of 2829 the message. 2831 4.4.1. Received Header Field 2833 When an SMTP server receives a message for delivery or further 2834 processing, it MUST insert trace (often referred to as "time stamp" 2835 or "Received" information) at the beginning of the message content, 2836 as discussed in Section 4.1.1.4. 2838 This line MUST be structured as follows: 2840 * The FROM clause, which MUST be supplied in an SMTP environment, 2841 SHOULD contain both (1) the name of the source host as presented 2842 in the EHLO command and (2) an address literal containing the IP 2843 address of the source, determined from the TCP connection. 2845 * The ID clause MAY contain an "@" as suggested in RFC 822, but this 2846 is not required. 2848 * If the FOR clause appears, it MUST contain exactly one 2849 entry, even when multiple RCPT commands have been given. Multiple 2850 s raise some security issues and have been deprecated, see 2851 Section 7.2. 2853 An Internet mail program MUST NOT change or delete a Received: line 2854 that was previously added to the message header section. SMTP 2855 servers MUST prepend Received lines to messages; they MUST NOT change 2856 the order of existing lines or insert Received lines in any other 2857 location. 2859 As the Internet grows, comparability of Received header fields is 2860 important for detecting problems, especially slow relays. SMTP 2861 servers that create Received header fields SHOULD use explicit 2862 offsets in the dates (e.g., -0800), rather than time zone names of 2863 any type. Local time (with an offset) SHOULD be used rather than UT 2864 when feasible. This formulation allows slightly more information 2865 about local circumstances to be specified. If UT is needed, the 2866 receiver need merely do some simple arithmetic to convert the values. 2867 Use of UT loses information about the time zone-location of the 2868 server. If it is desired to supply a time zone name, it SHOULD be 2869 included in a comment. 2871 When the delivery SMTP server makes the "final delivery" of a 2872 message, it inserts a return-path line at the beginning of the mail 2873 data. This use of return-path is required; mail systems MUST support 2874 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2876 has left the SMTP environment. Normally, this would mean it had been 2877 delivered to the destination user or an associated mail drop, but in 2878 some cases it may be further processed and transmitted by another 2879 mail system. 2881 It is possible for the mailbox in the return path to be different 2882 from the actual sender's mailbox, for example, if error responses are 2883 to be delivered to a special error handling mailbox rather than to 2884 the message sender. When mailing lists are involved, this 2885 arrangement is common and useful as a means of directing errors to 2886 the list maintainer rather than the message originator. 2888 The text above implies that the final mail data will begin with a 2889 return path line, followed by one or more time stamp lines. These 2890 lines will be followed by the rest of the mail data: first the 2891 balance of the mail header section and then the body (RFC 5322 [12]). 2893 It is sometimes difficult for an SMTP server to determine whether or 2894 not it is making final delivery since forwarding or other operations 2895 may occur after the message is accepted for delivery. Consequently, 2896 any further (forwarding, gateway, or relay) systems MAY remove the 2897 return path and rebuild the MAIL command as needed to ensure that 2898 exactly one such line appears in a delivered message. 2900 A message-originating SMTP system SHOULD NOT send a message that 2901 already contains a Return-path header field. SMTP servers performing 2902 a relay function MUST NOT inspect the message data, and especially 2903 not to the extent needed to determine if Return-path header fields 2904 are present. SMTP servers making final delivery MAY remove Return- 2905 path header fields before adding their own. 2907 The primary purpose of the Return-path is to designate the address to 2908 which messages indicating non-delivery or other mail system failures 2909 are to be sent. For this to be unambiguous, exactly one return path 2910 SHOULD be present when the message is delivered. Systems using RFC 2911 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2912 address, associated with the transport envelope, to which error 2913 reports (e.g., non-delivery messages) should be sent. 2915 Historical note: Text in RFC 822 that appears to contraindicate the 2916 use of the Return-path header field (or the envelope reverse-path 2917 address from the MAIL command) if the destination for error messages 2918 is not applicable on the Internet. The reverse-path address (as 2919 copied into the Return-path) MUST be used as the target of any mail 2920 containing delivery error messages. 2922 In particular: 2924 * a gateway from SMTP -> elsewhere SHOULD insert a return-path 2925 header field, unless it is known that the "elsewhere" transport 2926 also uses Internet domain addresses and maintains the envelope 2927 sender address separately. 2929 * a gateway from elsewhere -> SMTP SHOULD delete any return-path 2930 header field present in the message, and either copy that 2931 information to the SMTP envelope or combine it with information 2932 present in the envelope of the other transport system to construct 2933 the reverse-path argument to the MAIL command in the SMTP 2934 envelope. 2936 The server must give special treatment to cases in which the 2937 processing following the end of mail data indication is only 2938 partially successful. This could happen if, after accepting several 2939 recipients and the mail data, the SMTP server finds that the mail 2940 data could be successfully delivered to some, but not all, of the 2941 recipients. In such cases, the response to the DATA command MUST be 2942 an OK reply. However, the SMTP server MUST compose and send an 2943 "undeliverable mail" notification message to the originator of the 2944 message. 2946 // [JcK/Alexey 20211104] The following paragraph does not seem to 2947 // belong in this section. Where should it be moved? 2948 A single notification listing all of the failed recipients or 2949 separate notification messages MUST be sent for each failed 2950 recipient. For economy of processing by the sender, the former 2951 SHOULD be used when possible. All notification messages about 2952 undeliverable mail MUST be sent using the MAIL command and MUST use a 2953 null return path as discussed in Section 3.6. 2955 The time stamp line and the return path line are formally defined as 2956 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2957 [12]): 2959 Return-path-line = "Return-Path:" FWS Reverse-path 2961 Time-stamp-line = "Received:" FWS Stamp 2963 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2964 FWS date-time 2965 ; where "date-time" is as defined in RFC 5322 [12] 2966 ; but the "obs-" forms, especially two-digit 2967 ; years, are prohibited in SMTP and MUST NOT be used. 2969 From-domain = "FROM" FWS Extended-Domain 2971 By-domain = CFWS "BY" FWS Extended-Domain 2973 Extended-Domain = Domain / 2974 ( Domain FWS "(" TCP-info ")" ) / 2975 ( address-literal FWS "(" TCP-info ")" ) 2977 TCP-info = address-literal / ( Domain FWS address-literal ) 2978 ; Information derived by server from TCP connection 2979 ; not client EHLO. 2981 Opt-info = [Via] [With] [ID] [For] 2982 [Additional-Registered-Clauses] 2984 Via = CFWS "VIA" FWS Link 2986 With = CFWS "WITH" FWS Protocol 2988 ID = CFWS "ID" FWS ( Atom / msg-id ) 2989 ; msg-id is defined in RFC 5322 [12] 2991 For = CFWS "FOR" FWS ( Path / Mailbox ) 2993 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 2994 // [5321bis] 5321 errata #1683, 20090215, 2995 ; Additional standard clauses may be added in this 2996 ; location by future standards and registration with 2997 ; IANA. SMTP servers SHOULD NOT use unregistered 2998 ; names. See Section 8. 3000 Link = "TCP" / Addtl-Link 3002 Addtl-Link = Atom 3003 ; Additional standard names for links are 3004 ; registered with the Internet Assigned Numbers 3005 ; Authority (IANA). "Via" is primarily of value 3006 ; with non-Internet transports. SMTP servers 3007 ; SHOULD NOT use unregistered names. 3009 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 3011 Addtl-Protocol = Atom 3012 ; Additional standard names for protocols are 3013 ; registered with the Internet Assigned Numbers 3014 ; Authority (IANA) in the "mail parameters" 3015 ; registry [8]. SMTP servers SHOULD NOT 3016 ; use unregistered names. 3018 4.5. Additional Implementation Issues 3020 4.5.1. Minimum Implementation 3022 In order to make SMTP workable, the following minimum implementation 3023 MUST be provided by all receivers. The following commands MUST be 3024 supported to conform to this specification: 3026 EHLO 3027 HELO 3028 MAIL 3029 RCPT 3030 DATA 3031 RSET 3032 NOOP 3033 QUIT 3034 VRFY 3036 Any system that includes an SMTP server supporting mail relaying or 3037 delivery MUST support the reserved mailbox "postmaster" as a case- 3038 insensitive local name. This postmaster address is not strictly 3039 necessary if the server always returns 554 on connection opening (as 3040 described in Section 3.1). The requirement to accept mail for 3041 postmaster implies that RCPT commands that specify a mailbox for 3042 postmaster at any of the domains for which the SMTP server provides 3043 mail service, as well as the special case of "RCPT TO:" 3044 (with no domain specification), MUST be supported. 3046 SMTP systems are expected to make every reasonable effort to accept 3047 mail directed to Postmaster from any other system on the Internet. 3048 In extreme cases -- such as to contain a denial of service attack or 3049 other breach of security -- an SMTP server may block mail directed to 3050 Postmaster. However, such arrangements SHOULD be narrowly tailored 3051 so as to avoid blocking messages that are not part of such attacks. 3053 4.5.2. Transparency 3055 Without some provision for data transparency, the character sequence 3056 "." ends the mail text and cannot be sent by the user. 3057 In general, users are not aware of such "forbidden" sequences. To 3058 allow all user composed text to be transmitted transparently, the 3059 following procedures are used: 3061 * Before sending a line of mail text, the SMTP client checks the 3062 first character of the line. If it is a period, one additional 3063 period is inserted at the beginning of the line. 3065 * When a line of mail text is received by the SMTP server, it checks 3066 the line. If the line is composed of a single period, it is 3067 treated as the end of mail indicator. If the first character is a 3068 period and there are other characters on the line, the first 3069 character is deleted. 3071 The mail data may contain any of the 128 ASCII characters. All 3072 characters are to be delivered to the recipient's mailbox, including 3073 spaces, vertical and horizontal tabs, and other control characters. 3074 If the transmission channel provides an 8-bit byte (octet) data 3075 stream, the 7-bit ASCII codes are transmitted, right justified, in 3076 the octets, with the high-order bits cleared to zero. See 3077 Section 3.6 for special treatment of these conditions in SMTP systems 3078 serving a relay function. 3080 In some systems, it may be necessary to transform the data as it is 3081 received and stored. This may be necessary for hosts that use a 3082 different character set than ASCII as their local character set, that 3083 store data in records rather than strings, or which use special 3084 character sequences as delimiters inside mailboxes. If such 3085 transformations are necessary, they MUST be reversible, especially if 3086 they are applied to mail being relayed. 3088 4.5.3. Sizes and Timeouts 3090 4.5.3.1. Size Limits and Minimums 3092 There are several objects that have required minimum/maximum sizes. 3093 Every implementation MUST be able to receive objects of at least 3094 these sizes. Objects larger than these sizes SHOULD be avoided when 3095 possible. However, some Internet mail constructs such as encoded 3096 X.400 addresses (RFC 2156 [27]) will often require larger objects. 3097 Clients MAY attempt to transmit these, but MUST be prepared for a 3098 server to reject them if they cannot be handled by it. To the 3099 maximum extent possible, implementation techniques that impose no 3100 limits on the length of these objects should be used. 3102 Extensions to SMTP may involve the use of characters that occupy more 3103 than a single octet each. This section therefore specifies lengths 3104 in octets where absolute lengths, rather than character counts, are 3105 intended. 3107 // [5321bis] [[Note in Draft: Klensin 20191126: Given the controversy 3108 // on the SMTP mailing list between 20191123 and now about maximum 3109 // lengths, is the above adequate or is further tuning of the limit 3110 // text below needed? 3112 4.5.3.1.1. Local-part 3114 The maximum total length of a user name or other local-part is 64 3115 octets. 3117 4.5.3.1.2. Domain 3119 The maximum total length of a domain name or number is 255 octets. 3121 4.5.3.1.3. Path 3123 The maximum total length of a reverse-path or forward-path is 256 3124 octets (including the punctuation and element separators). 3126 4.5.3.1.4. Command Line 3128 The maximum total length of a command line including the command word 3129 and the is 512 octets. SMTP extensions may be used to 3130 increase this limit. 3132 4.5.3.1.5. Reply Line 3134 The maximum total length of a reply line including the reply code and 3135 the is 512 octets. More information may be conveyed through 3136 multiple-line replies. 3138 4.5.3.1.6. Text Line 3140 The maximum total length of a text line including the is 1000 3141 octets (not counting the leading dot duplicated for transparency). 3142 This number may be increased by the use of SMTP Service Extensions. 3144 4.5.3.1.7. Message Content 3146 The maximum total length of a message content (including any message 3147 header section as well as the message body) MUST BE at least 64K 3148 octets. Since the introduction of Internet Standards for multimedia 3149 mail (RFC 2045 [25]), message lengths on the Internet have grown 3150 dramatically, and message size restrictions should be avoided if at 3151 all possible. SMTP server systems that must impose restrictions 3152 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3153 SMTP client systems that will send large messages SHOULD utilize it 3154 when possible. 3156 4.5.3.1.8. Recipient Buffer 3158 The minimum total number of recipients that MUST be buffered is 100 3159 recipients. Rejection of messages (for excessive recipients) with 3160 fewer than 100 RCPT commands is a violation of this specification. 3161 The general principle that relaying SMTP server MUST NOT, and 3162 delivery SMTP servers SHOULD NOT, perform validation tests on message 3163 header fields suggests that messages SHOULD NOT be rejected based on 3164 the total number of recipients shown in header fields. A server that 3165 imposes a limit on the number of recipients MUST behave in an orderly 3166 fashion, such as rejecting additional addresses over its limit rather 3167 than silently discarding addresses previously accepted. A client 3168 that needs to deliver a message containing over 100 RCPT commands 3169 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3170 server declines to accept more than 100 recipients in a single 3171 message. 3173 4.5.3.1.9. Treatment When Limits Exceeded 3175 Errors due to exceeding these limits may be reported by using the 3176 reply codes. Some examples of reply codes are: 3178 500 Line too long. 3180 or 3182 501 Path too long 3184 or 3186 452 Too many recipients (see below) 3188 or 3190 552 Too much mail data (historically also used for too many 3191 recipients (see below). 3193 4.5.3.1.10. Too Many Recipients Code 3195 RFC 821 [3] incorrectly listed the error where an SMTP server 3196 exhausts its implementation limit on the number of RCPT commands 3197 ("too many recipients") as having reply code 552. The correct reply 3198 code for this condition is 452. At the time RFC 5321 was written, 3199 the use of response code 552 by servers was sufficiently common that 3200 client implementation were advised to simply treat it as if 452 had 3201 been sent. That advice is no longer necessary or useful. 3203 When a conforming SMTP server encounters this condition, it has at 3204 least 100 successful RCPT commands in its recipient buffer. If the 3205 server is able to accept the message, then at least these 100 3206 addresses will be removed from the SMTP client's queue. When the 3207 client attempts retransmission of those addresses that received 452 3208 responses, at least 100 of these will be able to fit in the SMTP 3209 server's recipient buffer. Each retransmission attempt that is able 3210 to deliver anything will be able to dispose of at least 100 of these 3211 recipients. 3213 If an SMTP server has an implementation limit on the number of RCPT 3214 commands and this limit is exhausted, it MUST use a response code of 3215 452. If the server has a configured site-policy limitation on the 3216 number of RCPT commands, it MAY instead use a 5yz response code. In 3217 particular, if the intent is to prohibit messages with more than a 3218 site-specified number of recipients, rather than merely limit the 3219 number of recipients in a given mail transaction, it would be 3220 reasonable to return a 503 response to any DATA command received 3221 subsequent to the 452 code or to simply return the 503 after DATA 3222 without returning any previous negative response. 3224 4.5.3.2. Timeouts 3226 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3227 command timeouts rather than somehow trying to time the entire mail 3228 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3229 without recompiling the SMTP code. To implement this, a timer is set 3230 for each SMTP command and for each buffer of the data transfer. The 3231 latter means that the overall timeout is inherently proportional to 3232 the size of the message. 3234 Based on extensive experience with busy mail-relay hosts, the minimum 3235 per-command timeout values SHOULD be as follows: 3237 4.5.3.2.1. Initial 220 Message: 5 Minutes 3239 An SMTP client process needs to distinguish between a failed TCP 3240 connection and a delay in receiving the initial 220 greeting message. 3241 Many SMTP servers accept a TCP connection but delay delivery of the 3242 220 message until their system load permits more mail to be 3243 processed. 3245 4.5.3.2.2. MAIL Command: 5 Minutes 3247 4.5.3.2.3. RCPT Command: 5 Minutes 3249 A longer timeout is required if processing of mailing lists and 3250 aliases is not deferred until after the message was accepted. 3252 4.5.3.2.4. DATA Initiation: 2 Minutes 3254 This is while awaiting the "354 Start Input" reply to a DATA command. 3256 4.5.3.2.5. Data Block: 3 Minutes 3258 This is while awaiting the completion of each TCP SEND call 3259 transmitting a chunk of data. 3261 4.5.3.2.6. DATA Termination: 10 Minutes. 3263 This is while awaiting the "250 OK" reply. When the receiver gets 3264 the final period terminating the message data, it typically performs 3265 processing to deliver the message to a user mailbox. A spurious 3266 timeout at this point would be very wasteful and would typically 3267 result in delivery of multiple copies of the message, since it has 3268 been successfully sent and the server has accepted responsibility for 3269 delivery. See Section 6.1 for additional discussion. 3271 4.5.3.2.7. Server Timeout: 5 Minutes. 3273 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3274 is awaiting the next command from the sender. 3276 4.5.4. Retry Strategies 3278 The common structure of a host SMTP implementation includes user 3279 mailboxes, one or more areas for queuing messages in transit, and one 3280 or more daemon processes for sending and receiving mail. The exact 3281 structure will vary depending on the needs of the users on the host 3282 and the number and size of mailing lists supported by the host. We 3283 describe several optimizations that have proved helpful, particularly 3284 for mailers supporting high traffic levels. 3286 Any queuing strategy MUST include timeouts on all activities on a 3287 per-command basis. A queuing strategy MUST NOT send error messages 3288 in response to error messages under any circumstances. 3290 4.5.4.1. Sending Strategy 3292 The general model for an SMTP client is one or more processes that 3293 periodically attempt to transmit outgoing mail. In a typical system, 3294 the program that composes a message has some method for requesting 3295 immediate attention for a new piece of outgoing mail, while mail that 3296 cannot be transmitted immediately MUST be queued and periodically 3297 retried by the sender. A mail queue entry will include not only the 3298 message itself but also the envelope information. 3300 The sender MUST delay retrying a particular destination after one 3301 attempt has failed. In general, the retry interval SHOULD be at 3302 least 30 minutes; however, more sophisticated and variable strategies 3303 will be beneficial when the SMTP client can determine the reason for 3304 non-delivery. 3306 Retries continue until the message is transmitted or the sender gives 3307 up; the give-up time generally needs to be at least 4-5 days. It MAY 3308 be appropriate to set a shorter maximum number of retries for non- 3309 delivery notifications and equivalent error messages than for 3310 standard messages. The parameters to the retry algorithm MUST be 3311 configurable. 3313 A client SHOULD keep a list of hosts it cannot reach and 3314 corresponding connection timeouts, rather than just retrying queued 3315 mail items. 3317 Experience suggests that failures are typically transient (the target 3318 system or its connection has crashed), favoring a policy of two 3319 connection attempts in the first hour the message is in the queue, 3320 and then backing off to one every two or three hours. 3322 The SMTP client can shorten the queuing delay in cooperation with the 3323 SMTP server. For example, if mail is received from a particular 3324 address, it is likely that mail queued for that host can now be sent. 3325 Application of this principle may, in many cases, eliminate the 3326 requirement for an explicit "send queues now" function such as ETRN, 3327 RFC 1985 [24]. 3329 The strategy may be further modified as a result of multiple 3330 addresses per host (see below) to optimize delivery time versus 3331 resource usage. 3333 An SMTP client may have a large queue of messages for each 3334 unavailable destination host. If all of these messages were retried 3335 in every retry cycle, there would be excessive Internet overhead and 3336 the sending system would be blocked for a long period. Note that an 3337 SMTP client can generally determine that a delivery attempt has 3338 failed only after a timeout of several minutes, and even a one-minute 3339 timeout per connection will result in a very large delay if retries 3340 are repeated for dozens, or even hundreds, of queued messages to the 3341 same host. 3343 At the same time, SMTP clients SHOULD use great care in caching 3344 negative responses from servers. In an extreme case, if EHLO is 3345 issued multiple times during the same SMTP connection, different 3346 answers may be returned by the server. More significantly, 5yz 3347 responses to the MAIL command MUST NOT be cached. 3349 When a mail message is to be delivered to multiple recipients, and 3350 the SMTP server to which a copy of the message is to be sent is the 3351 same for multiple recipients, then only one copy of the message 3352 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3353 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3354 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3355 are very many addresses, a limit on the number of RCPT commands per 3356 MAIL command MAY be imposed. This efficiency feature SHOULD be 3357 implemented. 3359 Similarly, to achieve timely delivery, the SMTP client MAY support 3360 multiple concurrent outgoing mail transactions. However, some limit 3361 may be appropriate to protect the host from devoting all its 3362 resources to mail. 3364 4.5.4.2. Receiving Strategy 3366 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3367 port (specified by IANA as port 25) at all times. This requires the 3368 support of multiple incoming TCP connections for SMTP. Some limit 3369 MAY be imposed, but servers that cannot handle more than one SMTP 3370 transaction at a time are not in conformance with the intent of this 3371 specification. 3373 As discussed above, when the SMTP server receives mail from a 3374 particular host address, it could activate its own SMTP queuing 3375 mechanisms to retry any mail pending for that host address. 3377 4.5.5. Messages with a Null Reverse-Path 3379 There are several types of notification messages that are required by 3380 existing and proposed Standards to be sent with a null reverse-path, 3381 namely non-delivery notifications as discussed in Section 3.6.1 and 3382 Section 3.6.2, other kinds of Delivery Status Notifications (DSNs, 3383 RFC 3461 [34]), and Message Disposition Notifications (MDNs, RFC 8098 3384 [37]). All of these kinds of messages are notifications about a 3385 previous message, and they are sent to the reverse-path of the 3386 previous mail message. (If the delivery of such a notification 3387 message fails, that usually indicates a problem with the mail system 3388 of the host to which the notification message is addressed. For this 3389 reason, at some hosts the MTA is set up to forward such failed 3390 notification messages to someone who is able to fix problems with the 3391 mail system, e.g., via the postmaster alias.) 3393 All other types of messages (i.e., any message which is not required 3394 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3395 with a valid, non-null reverse-path. 3397 Implementers of automated email processors should be careful to make 3398 sure that the various kinds of messages with a null reverse-path are 3399 handled correctly. In particular, such systems SHOULD NOT reply to 3400 messages with a null reverse-path, and they SHOULD NOT add a non-null 3401 reverse-path, or change a null reverse-path to a non-null one, to 3402 such messages when forwarding. 3404 5. Address Resolution and Mail Handling 3405 5.1. Locating the Target Host 3407 Once an SMTP client lexically identifies a domain to which mail will 3408 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3409 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3410 [4]. The names are required to be fully-qualified domain names 3411 (FQDNs) as discussed in Section 2.3.5. 3413 The lookup first attempts to locate an MX record associated with the 3414 name. If a CNAME record is found, the resulting name is processed as 3415 if it were the initial name. If a non-existent domain error is 3416 returned, this situation MUST be reported as an error. If a 3417 temporary error is returned, the message MUST be queued and retried 3418 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3419 the address is treated as if it was associated with an implicit MX RR 3420 with a preference of 0, pointing to that host. If MX records are 3421 present, but none of them are usable, or the implicit MX is unusable, 3422 this situation MUST be reported as an error. 3424 When the lookup succeeds, the mapping can result in a list of 3425 alternative delivery addresses rather than a single address. This 3426 can be due to multiple MX records, multihoming, or both. To provide 3427 reliable mail transmission, the SMTP client MUST be able to try (and 3428 be prepared to retry) each of the relevant addresses in this list in 3429 order (see below), until a delivery attempt succeeds. However, as 3430 discussed more generally in Section 7.8 there MAY also be a 3431 configurable limit on the number of alternate addresses that can be 3432 tried. In any case, the SMTP client SHOULD try at least two 3433 addresses. 3435 If one or more MX RRs are found for a given name, SMTP systems MUST 3436 NOT utilize any address RRs associated with that name unless they are 3437 located using the MX RRs; the "implicit MX" rule above applies only 3438 if there are no MX records present. If MX records are present, but 3439 none of them are usable, this situation MUST be reported as an error. 3441 When a domain name associated with an MX RR is looked up and the 3442 associated data field obtained, the data field of that response MUST 3443 contain a domain name that conforms to the specifications of 3444 Section 2.3.5. 3445 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3446 unfolds, some additional text may be in order here (20140718)]] 3447 That domain name, when queried, MUST return at least one address 3448 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3449 server to which the message should be directed. Any other response, 3450 specifically including a value that will return a CNAME record when 3451 queried, lies outside the scope of this Standard. The prohibition on 3452 labels in the data that resolve to CNAMEs is discussed in more detail 3453 in RFC 2181, Section 10.3 [28]. 3455 Two types of information are used to rank the host addresses: 3456 multiple MX records, and multihomed hosts. 3458 MX records contain a numerical preference indication that MUST be 3459 used in sorting if more than one such record appears. Lower numbers 3460 are more preferred than higher ones. The sender-SMTP MUST inspect 3461 the list for any of the names or addresses by which it might be known 3462 in mail transactions. If a matching record is found, all records at 3463 that preference level and higher-numbered ones MUST be discarded from 3464 consideration. If there are no records left at that point, it is an 3465 error condition, and a 5yz reply code generated (terminating the mail 3466 transaction) or the message MUST be returned as undeliverable. If 3467 there is a single MX record at the most-preferred preference label, 3468 the data field associated with that record is used as the next 3469 destination. Otherwise, if there are multiple records with the same 3470 preference and there is no clear reason to favor one (e.g., by 3471 recognition of an easily reached address), then the sender-SMTP MUST 3472 randomize them to spread the load across multiple mail exchangers for 3473 a specific organization. 3475 The destination host (from either the data field of the preferred MX 3476 record of from an address records fount in an implicit MX) may be 3477 multihomed. In those cases the domain name resolver will return a 3478 list of alternative IP addresses. It is the responsibility of the 3479 domain name resolver interface to have ordered this list by 3480 decreasing preference if necessary, and the SMTP sender MUST try them 3481 in the order presented. 3483 Although the capability to try multiple alternative addresses is 3484 required, specific installations may want to limit or disable the use 3485 of alternative addresses. The question of whether a sender should 3486 attempt retries using the different addresses of a multihomed host 3487 has been controversial. The main argument for using the multiple 3488 addresses is that it maximizes the likelihood of timely delivery, and 3489 indeed sometimes the likelihood of any delivery; the counter-argument 3490 is that it may result in unnecessary resource use. Note that 3491 resource use is also strongly determined by the sending strategy 3492 discussed in Section 4.5.4.1. 3494 If an SMTP server receives a message with a destination for which it 3495 is a designated Mail eXchanger, it MAY relay the message (potentially 3496 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3497 final delivery of the message, or hand it off using some mechanism 3498 outside the SMTP-provided transport environment. Of course, neither 3499 of the latter require that the list of MX records be examined 3500 further. 3502 If it determines that it should relay the message without rewriting 3503 the address, it MUST process the MX records as described above to 3504 determine candidates for delivery. 3506 5.2. IPv6 and MX Records 3508 In the contemporary Internet, SMTP clients and servers may be hosted 3509 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3510 compatible with either version of the Internet Protocol. The host 3511 domains to which MX records point may, consequently, contain "A RR"s 3512 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3513 3974 [39] discusses some operational experience in mixed 3514 environments, it was not comprehensive enough to justify 3515 standardization, and some of its recommendations appear to be 3516 inconsistent with this specification. The appropriate actions to be 3517 taken either will depend on local circumstances, such as performance 3518 of the relevant networks and any conversions that might be necessary, 3519 or will be obvious (e.g., an IPv6-only client need not attempt to 3520 look up A RRs or attempt to reach IPv4-only servers). Designers of 3521 SMTP implementations that might run in IPv6 or dual-stack 3522 environments should study the procedures above, especially the 3523 comments about multihomed hosts, and, preferably, provide mechanisms 3524 to facilitate operational tuning and mail interoperability between 3525 IPv4 and IPv6 systems while considering local circumstances. 3527 6. Problem Detection and Handling 3529 6.1. Reliable Delivery and Replies by Email 3531 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3532 message in response to DATA), it is accepting responsibility for 3533 delivering or relaying the message. It must take this responsibility 3534 seriously. It MUST NOT lose the message for frivolous reasons, such 3535 as because the host later crashes or because of a predictable 3536 resource shortage. Some reasons that are not considered frivolous 3537 are discussed in the next subsection and in Section 7.8. 3539 If there is a delivery failure after acceptance of a message, the 3540 receiver-SMTP MUST formulate and mail a notification message. This 3541 notification MUST be sent using a null ("<>") reverse-path in the 3542 envelope. The recipient of this notification MUST be the address 3543 from the envelope return path (or the Return-Path: line). However, 3544 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3545 notification. Obviously, nothing in this section can or should 3546 prohibit local decisions (i.e., as part of the same system 3547 environment as the receiver-SMTP) to log or otherwise transmit 3548 information about null address events locally if that is desired. 3550 Some delivery failures after the message is accepted by SMTP will be 3551 unavoidable. For example, it may be impossible for the receiving 3552 SMTP server to validate all the delivery addresses in RCPT command(s) 3553 due to a "soft" domain system error, because the target is a mailing 3554 list (see earlier discussion of RCPT), or because the server is 3555 acting as a relay and has no immediate access to the delivering 3556 system. 3558 To avoid receiving duplicate messages as the result of timeouts, a 3559 receiver-SMTP MUST seek to minimize the time required to respond to 3560 the final . end of data indicator. See RFC 1047 [17] for 3561 a discussion of this problem. 3563 6.2. Unwanted, Unsolicited, and "Attack" Messages 3565 Utility and predictability of the Internet mail system requires that 3566 messages that can be delivered should be delivered, regardless of any 3567 syntax or other faults associated with those messages and regardless 3568 of their content. If they cannot be delivered, and cannot be 3569 rejected by the SMTP server during the SMTP transaction, they should 3570 be "bounced" (returned with non-delivery notification messages) as 3571 described above. In today's world, in which many SMTP server 3572 operators have discovered that the quantity of undesirable bulk email 3573 vastly exceeds the quantity of desired mail and in which accepting a 3574 message may trigger additional undesirable traffic by providing 3575 verification of the address, those principles may not be practical. 3577 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3578 without notification of the sender is permitted in practice. 3579 However, it is extremely dangerous and violates a long tradition and 3580 community expectations that mail is either delivered or returned. If 3581 silent message-dropping is misused, it could easily undermine 3582 confidence in the reliability of the Internet's mail systems. So 3583 silent dropping of messages should be considered only in those cases 3584 where there is very high confidence that the messages are seriously 3585 fraudulent or otherwise inappropriate. 3587 To stretch the principle of delivery if possible even further, it may 3588 be a rational policy to not deliver mail that has an invalid return 3589 address, although the history of the network is that users are 3590 typically better served by delivering any message that can be 3591 delivered. Reliably determining that a return address is invalid can 3592 be a difficult and time-consuming process, especially if the putative 3593 sending system is not directly accessible or does not fully and 3594 accurately support VRFY and, even if a "drop messages with invalid 3595 return addresses" policy is adopted, it SHOULD be applied only when 3596 there is near-certainty that the return addresses are, in fact, 3597 invalid. 3599 Conversely, if a message is rejected because it is found to contain 3600 hostile content (a decision that is outside the scope of an SMTP 3601 server as defined in this document), rejection ("bounce") messages 3602 SHOULD NOT be sent unless the receiving site is confident that those 3603 messages will be usefully delivered. The preference and default in 3604 these cases is to avoid sending non-delivery messages when the 3605 incoming message is determined to contain hostile content. 3607 6.3. Loop Detection 3609 Simple counting of the number of "Received:" header fields in a 3610 message has proven to be an effective, although rarely optimal, 3611 method of detecting loops in mail systems. SMTP servers using this 3612 technique SHOULD use a large rejection threshold, normally at least 3613 100 Received entries. Whatever mechanisms are used, servers MUST 3614 contain provisions for detecting and stopping trivial loops. 3616 6.4. Compensating for Irregularities 3618 Unfortunately, variations, creative interpretations, and outright 3619 violations of Internet mail protocols do occur; some would suggest 3620 that they occur quite frequently. The debate as to whether a well- 3621 behaved SMTP receiver or relay should reject a malformed message, 3622 attempt to pass it on unchanged, or attempt to repair it to increase 3623 the odds of successful delivery (or subsequent reply) began almost 3624 with the dawn of structured network mail and shows no signs of 3625 abating. Advocates of rejection claim that attempted repairs are 3626 rarely completely adequate and that rejection of bad messages is the 3627 only way to get the offending software repaired. Advocates of 3628 "repair" or "deliver no matter what" argue that users prefer that 3629 mail go through it if at all possible and that there are significant 3630 market pressures in that direction. In practice, these market 3631 pressures may be more important to particular vendors than strict 3632 conformance to the standards, regardless of the preference of the 3633 actual developers. 3635 The problems associated with ill-formed messages were exacerbated by 3636 the introduction of the split-UA mail reading protocols (Post Office 3637 Protocol (POP) version 2 [14], Post Office Protocol (POP) version 3 3638 [23], IMAP version 2 [19], and PCMAIL [18]). These protocols 3639 encouraged the use of SMTP as a posting (message submission) 3640 protocol, and SMTP servers as relay systems for these client hosts 3641 (which are often only intermittently connected to the Internet). 3642 Historically, many of those client machines lacked some of the 3643 mechanisms and information assumed by SMTP (and indeed, by the mail 3644 format protocol, RFC 822 [13]). Some could not keep adequate track 3645 of time; others had no concept of time zones; still others could not 3646 identify their own names or addresses; and, of course, none could 3647 satisfy the assumptions that underlay RFC 822's conception of 3648 authenticated addresses. 3650 In response to these weak SMTP clients, many SMTP systems now 3651 complete messages that are delivered to them in incomplete or 3652 incorrect form. This strategy is generally considered appropriate 3653 when the server can identify or authenticate the client, and there 3654 are prior agreements between them. By contrast, there is at best 3655 great concern about fixes applied by a relay or delivery SMTP server 3656 that has little or no knowledge of the user or client machine. Many 3657 of these issues are addressed by using a separate protocol, such as 3658 that defined in RFC 6409 [41], for message submission, rather than 3659 using originating SMTP servers for that purpose. 3661 The following changes to a message being processed MAY be applied 3662 when necessary by an originating SMTP server, or one used as the 3663 target of SMTP as an initial posting (message submission) protocol: 3665 * Addition of a message-id field when none appears 3667 * Addition of a date, time, or time zone when none appears 3669 * Correction of addresses to proper FQDN format 3670 The less information the server has about the client, the less likely 3671 these changes are to be correct and the more caution and conservatism 3672 should be applied when considering whether or not to perform fixes 3673 and how. These changes MUST NOT be applied by an SMTP server that 3674 provides an intermediate relay function. 3676 In all cases, properly operating clients supplying correct 3677 information are preferred to corrections by the SMTP server. In all 3678 cases, documentation SHOULD be provided in trace header fields and/or 3679 header field comments for actions performed by the servers. 3681 7. Security Considerations 3683 7.1. Mail Security and Spoofing 3685 SMTP mail is inherently insecure in that it is feasible for even 3686 fairly casual users to negotiate directly with receiving and relaying 3687 SMTP servers and create messages that will trick a naive recipient 3688 into believing that they came from somewhere else. Constructing such 3689 a message so that the "spoofed" behavior cannot be detected by an 3690 expert is somewhat more difficult, but not sufficiently so as to be a 3691 deterrent to someone who is determined and knowledgeable. 3692 Consequently, as knowledge of Internet mail increases, so does the 3693 knowledge that SMTP mail inherently cannot be authenticated, or 3694 integrity checks provided, at the transport level. Real mail 3695 security lies only in end-to-end methods involving the message 3696 bodies, such as those that use digital signatures (see RFC 1847 [21] 3697 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [42] or Secure/ 3698 Multipurpose Internet Mail Extensions (S/MIME) in RFC 8551 [38]). 3700 Various protocol extensions and configuration options that provide 3701 authentication at the transport level (e.g., from an SMTP client to 3702 an SMTP server) improve somewhat on the traditional situation 3703 described above. However, in general, they only authenticate one 3704 server to another rather than a chain of relays and servers, much 3705 less authenticating users or user machines. Consequently, unless 3706 they are accompanied by careful handoffs of responsibility in a 3707 carefully designed trust environment, they remain inherently weaker 3708 than end-to-end mechanisms that use digitally signed messages rather 3709 than depending on the integrity of the transport system. 3711 Efforts to make it more difficult for users to set envelope return 3712 path and header "From" fields to point to valid addresses other than 3713 their own are largely misguided: they frustrate legitimate 3714 applications in which mail is sent by one user on behalf of another, 3715 in which error (or normal) replies should be directed to a special 3716 address, or in which a single message is sent to multiple recipients 3717 on different hosts. (Systems that provide convenient ways for users 3718 to alter these header fields on a per-message basis should attempt to 3719 establish a primary and permanent mailbox address for the user so 3720 that Sender header fields within the message data can be generated 3721 sensibly.) 3723 This specification does not further address the authentication issues 3724 associated with SMTP other than to advocate that useful functionality 3725 not be disabled in the hope of providing some small margin of 3726 protection against a user who is trying to fake mail. 3728 7.2. "Blind" Copies 3730 Addresses that do not appear in the message header section may appear 3731 in the RCPT commands to an SMTP server for a number of reasons. The 3732 two most common involve the use of a mailing address as a "list 3733 exploder" (a single address that resolves into multiple addresses) 3734 and the appearance of "blind copies". Especially when more than one 3735 RCPT command is present, and in order to avoid defeating some of the 3736 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3737 the full set of RCPT command arguments into the header section, 3738 either as part of trace header fields or as informational or private- 3739 extension header fields. 3740 // [rfc5321bis] [[Note in draft - Suggestion from 20070124 that got 3741 // lost: delete "especially" and "the full set of" -- copying the 3742 // first one can be as harmful as copying all of them, at least 3743 // without verifying that the addresses do appear in the headers. 3744 // See G.7.9 and ticket #15. 3745 Because this rule is often violated in practice, and cannot be 3746 enforced, sending SMTP systems that are aware of "bcc" use MAY find 3747 it helpful to send each blind copy as a separate message transaction 3748 containing only a single RCPT command. 3750 There is no inherent relationship between either "reverse" (from the 3751 MAIL command) or "forward" (RCPT) addresses in the SMTP transaction 3752 ("envelope") and the addresses in the header section. Receiving 3753 systems SHOULD NOT attempt to deduce such relationships and use them 3754 to alter the header section of the message for delivery. The popular 3755 "Apparently-to" header field is a violation of this principle as well 3756 as a common source of unintended information disclosure and SHOULD 3757 NOT be used. 3759 7.3. VRFY, EXPN, and Security 3761 As discussed in Section 3.5, individual sites may want to disable 3762 either or both of VRFY or EXPN for security reasons (see below). As 3763 a corollary to the above, implementations that permit this MUST NOT 3764 appear to have verified addresses that are not, in fact, verified. 3765 If a site disables these commands for security reasons, the SMTP 3766 server MUST return a 252 response, rather than a code that could be 3767 confused with successful or unsuccessful verification. 3769 Returning a 250 reply code with the address listed in the VRFY 3770 command after having checked it only for syntax violates this rule. 3771 Of course, an implementation that "supports" VRFY by always returning 3772 550 whether or not the address is valid is equally not in 3773 conformance. 3775 On the public Internet, the contents of mailing lists have become 3776 popular as an address information source for so-called "spammers." 3777 The use of EXPN to "harvest" addresses has increased as list 3778 administrators have installed protections against inappropriate uses 3779 of the lists themselves. However, VRFY and EXPN are still useful for 3780 authenticated users and within an administrative domain. For 3781 example, VRFY and EXPN are useful for performing internal audits of 3782 how email gets routed to check and to make sure no one is 3783 automatically forwarding sensitive mail outside the organization. 3784 Sites implementing SMTP authentication may choose to make VRFY and 3785 EXPN available only to authenticated requestors. Implementations 3786 SHOULD still provide support for EXPN, but sites SHOULD carefully 3787 evaluate the tradeoffs. 3789 Whether disabling VRFY provides any real marginal security depends on 3790 a series of other conditions. In many cases, RCPT commands can be 3791 used to obtain the same information about address validity. On the 3792 other hand, especially in situations where determination of address 3793 validity for RCPT commands is deferred until after the DATA command 3794 is received, RCPT may return no information at all, while VRFY is 3795 expected to make a serious attempt to determine validity before 3796 generating a response code (see discussion above). 3798 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3800 Before a client uses the 251 or 551 reply codes from a RCPT command 3801 to automatically update its future behavior (e.g., updating the 3802 user's address book), it should be certain of the server's 3803 authenticity. If it does not, it may be subject to a man in the 3804 middle attack. 3806 7.5. Information Disclosure in Announcements 3808 There has been an ongoing debate about the tradeoffs between the 3809 debugging advantages of announcing server type and version (and, 3810 sometimes, even server domain name) in the greeting response or in 3811 response to the HELP command and the disadvantages of exposing 3812 information that might be useful in a potential hostile attack. The 3813 utility of the debugging information is beyond doubt. Those who 3814 argue for making it available point out that it is far better to 3815 actually secure an SMTP server rather than hope that trying to 3816 conceal known vulnerabilities by hiding the server's precise identity 3817 will provide more protection. Sites are encouraged to evaluate the 3818 tradeoff with that issue in mind; implementations SHOULD minimally 3819 provide for making type and version information available in some way 3820 to other network hosts. 3822 7.6. Information Disclosure in Trace Fields 3824 In some circumstances, such as when mail originates from within a LAN 3825 whose hosts are not directly on the public Internet, trace (e.g., 3826 "Received") header fields produced in conformance with this 3827 specification may disclose host names and similar information that 3828 would not normally be available. This ordinarily does not pose a 3829 problem, but sites with special concerns about name disclosure should 3830 be aware of it. Also, the optional FOR clause should be supplied 3831 with caution or not at all when multiple recipients are involved lest 3832 it inadvertently disclose the identities of "blind copy" recipients 3833 to others. 3835 7.7. Information Disclosure in Message Forwarding 3837 As discussed in Section 3.4.1, use of the 251 or 551 reply codes to 3838 identify the replacement address associated with a mailbox may 3839 inadvertently disclose sensitive information. Sites that are 3840 concerned about those issues should ensure that they select and 3841 configure servers appropriately. 3843 7.8. Local Operational Requirements and Resistance to Attacks 3845 In recent years, there has been an increase of attacks on SMTP 3846 servers, either in conjunction with attempts to discover addresses 3847 for sending unsolicited messages or simply to make the servers 3848 inaccessible to others (i.e., as an application-level denial of 3849 service attack). There may also be important local circumstances 3850 that justify departures from some of the limits specified in this 3851 documents especially ones involving maximums or minimums. While the 3852 means of doing so are beyond the scope of this Standard, rational 3853 operational behavior requires that servers be permitted to detect 3854 such attacks and take action to defend themselves. For example, if a 3855 server determines that a large number of RCPT commands are being 3856 sent, most or all with invalid addresses, as part of such an attack, 3857 it would be reasonable for the server to close the connection after 3858 generating an appropriate number of 5yz (normally 550) replies. 3860 7.9. Scope of Operation of SMTP Servers 3862 It is a well-established principle that an SMTP server may refuse to 3863 accept mail for any operational or technical reason that makes sense 3864 to the site providing the server. However, cooperation among sites 3865 and installations makes the Internet possible. If sites take 3866 excessive advantage of the right to reject traffic, the ubiquity of 3867 email availability (one of the strengths of the Internet) will be 3868 threatened; considerable care should be taken and balance maintained 3869 if a site decides to be selective about the traffic it will accept 3870 and process. 3872 In recent years, use of the relay function through arbitrary sites 3873 has been used as part of hostile efforts to hide the actual origins 3874 of mail. Some sites have decided to limit the use of the relay 3875 function to known or identifiable sources, and implementations SHOULD 3876 provide the capability to perform this type of filtering. When mail 3877 is rejected for these or other policy reasons, a 550 code SHOULD be 3878 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3880 8. IANA Considerations 3882 IANA maintains three registries in support of this specification, all 3883 of which were created for RFC 2821 or earlier. This document expands 3884 the third one as specified below. The registry references listed are 3885 as of the time of publication; IANA does not guarantee the locations 3886 associated with the URLs. The registries are as follows: 3888 * The first, "Simple Mail Transfer Protocol (SMTP) Service 3889 Extensions" [49], consists of SMTP service extensions with the 3890 associated keywords, and, as needed, parameters and verbs. 3891 Entries may be made only for service extensions (and associated 3892 keywords, parameters, or verbs) that are defined in Standards- 3893 Track or Experimental RFCs specifically approved by the IESG for 3894 this purpose. 3896 * The second registry, "Address Literal Tags" [50], consists of 3897 "tags" that identify forms of domain literals other than those for 3898 IPv4 addresses (specified in RFC 821 and in this document). The 3899 initial entry in that registry is for IPv6 addresses (specified in 3900 this document). Additional literal types require standardization 3901 before being used; none are anticipated at this time. 3903 * The third, "Mail Transmission Types" [49], established by RFC 821 3904 and renewed by this specification, is a registry of link and 3905 protocol identifiers to be used with the "via" and "with" 3906 subclauses of the time stamp ("Received:" header field) described 3907 in Section 4.4. Link and protocol identifiers in addition to 3908 those specified in this document may be registered only by 3909 standardization or by way of an RFC-documented, IESG-approved, 3910 Experimental protocol extension. This name space is for 3911 identification and not limited in size: the IESG is encouraged to 3912 approve on the basis of clear documentation and a distinct method 3913 rather than preferences about the properties of the method itself. 3914 An additional subsection has been added to the "VIA link types" 3915 and "WITH protocol types" subsections of this registry to contain 3916 registrations of "Additional-registered-clauses" as described 3917 above. The registry will contain clause names, a description, a 3918 summary of the syntax of the associated String, and a reference. 3919 As new clauses are defined, they may, in principle, specify 3920 creation of their own registries if the Strings consist of 3921 reserved terms or keywords rather than less restricted strings. 3922 As with link and protocol identifiers, additional clauses may be 3923 registered only by standardization or by way of an RFC-documented, 3924 IESG-approved, Experimental protocol extension. The additional 3925 clause name space is for identification and is not limited in 3926 size: the IESG is encouraged to approve on the basis of clear 3927 documentation, actual use or strong signs that the clause will be 3928 used, and a distinct requirement rather than preferences about the 3929 properties of the clause itself. 3931 In addition, if additional trace header fields (i.e., in addition to 3932 Return-path and Received) are ever created, those trace fields MUST 3933 be added to the IANA registry established by BCP 90 (RFC 3864) [9] 3934 for use with RFC 5322 [12]. 3936 9. Acknowledgments 3938 Many people contributed to the development of RFCs 2821 and 5321. 3939 Those documents should be consulted for those acknowledgments. 3941 Neither this document nor RFCs 2821 or 5321 would have been possible 3942 without the many contribution and insights of the late Jon Postel. 3943 Those contributions of course include the original specification of 3944 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3945 appears in this document as do several of Jon's original examples 3946 that have been updated only as needed to reflect other changes in the 3947 specification. 3949 The following filed errata against RFC 5321 that were not rejected at 3950 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3951 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3952 Rodrigo Speller, Alessandro Vesely, and Brett Watson. Some of those 3953 individuals made additional suggestions after the EMAILCORE WG was 3954 initiated. In addition to the above, several of whom continued to 3955 make other suggestions, specific suggestions that led to corrections 3956 and improvements in early versions of the current specification were 3957 received from Dave Crocker, Ned Freed, Arnt Gulbrandsen, Tony Hansen, 3958 Barry Leiba, Ivar Lumi, Pete Resnick, Hector Santos, Paul Smith and 3959 others. 3961 chetti contributed an analysis that clarified the ABNF productions 3962 that implicitly reference other documents. 3964 The EMAILCORE Working Group was chartered in September 2020 with 3965 Alexey Melnikov and Seth Blank as co-chairs. Todd Herr replaced Seth 3966 Blank early in 2021. Without their leadership and technical 3967 contributions, this document would never have been completed. 3969 10. References 3971 10.1. Normative References 3973 [1] Bradner, S., "Key words for use in RFCs to Indicate 3974 Requirement Levels", BCP 14, RFC 2119, 3975 DOI 10.17487/RFC2119, March 1997, 3976 . 3978 [2] American National Standards Institute (formerly United 3979 States of America Standards Institute), "USA Code for 3980 Information Interchange", ANSI X3.4-1968, 1968. ANSI 3981 X3.4-1968 has been replaced by newer versions with slight 3982 modifications, but the 1968 version remains definitive for 3983 the Internet. 3985 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 3986 RFC 821, DOI 10.17487/RFC0821, August 1982, 3987 . 3989 [4] Mockapetris, P., "Domain names - implementation and 3990 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3991 November 1987, . 3993 [5] Braden, R., Ed., "Requirements for Internet Hosts - 3994 Application and Support", STD 3, RFC 1123, 3995 DOI 10.17487/RFC1123, October 1989, 3996 . 3998 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 3999 Extension for Message Size Declaration", STD 10, RFC 1870, 4000 DOI 10.17487/RFC1870, November 1995, 4001 . 4003 [7] Vaudreuil, G., "Enhanced Mail System Status Codes", 4004 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4005 . 4007 [8] Newman, C., "ESMTP and LMTP Transmission Types 4008 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 4009 . 4011 [9] Klyne, G., Nottingham, M., and J. Mogul, "Registration 4012 Procedures for Message Header Fields", BCP 90, RFC 3864, 4013 DOI 10.17487/RFC3864, September 2004, 4014 . 4016 [10] Hinden, R. and S. Deering, "IP Version 6 Addressing 4017 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 4018 2006, . 4020 [11] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 4021 Specifications: ABNF", STD 68, RFC 5234, 4022 DOI 10.17487/RFC5234, January 2008, 4023 . 4025 [12] Resnick, P., Ed., "Internet Message Format", RFC 5322, 4026 DOI 10.17487/RFC5322, October 2008, 4027 . 4029 10.2. Informative References 4031 [13] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 4032 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 4033 August 1982, . 4035 [14] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 4036 Reynolds, "Post Office Protocol: Version 2", RFC 937, 4037 DOI 10.17487/RFC0937, February 1985, 4038 . 4040 [15] Postel, J. and J. Reynolds, "File Transfer Protocol", 4041 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4042 . 4044 [16] Partridge, C., "Mail routing and the domain system", 4045 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4046 . 4048 [17] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4049 DOI 10.17487/RFC1047, February 1988, 4050 . 4052 [18] Lambert, M., "PCMAIL: A distributed mail system for 4053 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4054 1988, . 4056 [19] Crispin, M., "Interactive Mail Access Protocol: Version 4057 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4058 . 4060 [20] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4061 DOI 10.17487/RFC1846, September 1995, 4062 . 4064 [21] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4065 "Security Multiparts for MIME: Multipart/Signed and 4066 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4067 October 1995, . 4069 [22] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4070 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4071 DOI 10.17487/RFC1869, November 1995, 4072 . 4074 [23] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4075 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4076 . 4078 [24] De Winter, J., "SMTP Service Extension for Remote Message 4079 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4080 1996, . 4082 [25] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4083 Extensions (MIME) Part One: Format of Internet Message 4084 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4085 . 4087 [26] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4088 Part Three: Message Header Extensions for Non-ASCII Text", 4089 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4090 . 4092 [27] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4093 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4094 DOI 10.17487/RFC2156, January 1998, 4095 . 4097 [28] Elz, R. and R. Bush, "Clarifications to the DNS 4098 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4099 . 4101 [29] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4102 Word Extensions: Character Sets, Languages, and 4103 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4104 1997, . 4106 [30] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4107 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4108 . 4110 [31] Freed, N., "SMTP Service Extension for Command 4111 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4112 September 2000, . 4114 [32] Freed, N., "Behavior of and Requirements for Internet 4115 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4116 . 4118 [33] Vaudreuil, G., "SMTP Service Extensions for Transmission 4119 of Large and Binary MIME Messages", RFC 3030, 4120 DOI 10.17487/RFC3030, December 2000, 4121 . 4123 [34] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4124 Extension for Delivery Status Notifications (DSNs)", 4125 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4126 . 4128 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4129 for Delivery Status Notifications", RFC 3464, 4130 DOI 10.17487/RFC3464, January 2003, 4131 . 4133 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4134 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4135 . 4137 [37] Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition 4138 Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098, 4139 February 2017, . 4141 [38] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 4142 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 4143 Message Specification", RFC 8551, DOI 10.17487/RFC8551, 4144 April 2019, . 4146 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4147 in Mixed IPv4/v6 Environments", RFC 3974, 4148 DOI 10.17487/RFC3974, January 2005, 4149 . 4151 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4152 Resource Identifier (URI): Generic Syntax", STD 66, 4153 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4154 . 4156 [41] Gellens, R. and J. Klensin, "Message Submission for Mail", 4157 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 4158 . 4160 [42] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4161 Thayer, "OpenPGP Message Format", RFC 4880, 4162 DOI 10.17487/RFC4880, November 2007, 4163 . 4165 [43] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4166 Mail System Status Codes", BCP 138, RFC 5248, 4167 DOI 10.17487/RFC5248, June 2008, 4168 . 4170 [44] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4171 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4172 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4173 . 4175 [45] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4176 DOI 10.17487/RFC7504, June 2015, 4177 . 4179 [46] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4180 Record for Domains That Accept No Mail", RFC 7505, 4181 DOI 10.17487/RFC7505, June 2015, 4182 . 4184 [47] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 4185 DOI 10.17487/RFC5321, October 2008, 4186 . 4188 [48] Klensin, J.C., Ed., Murchison, K., Ed., and E. Sam, Ed., 4189 "Applicability Statement for IETF Core Email Protocols", 6 4190 August 2021, . 4193 [49] Internet Assigned Number Authority (IANA), "IANA Mail 4194 Parameters", 2007, 4195 . 4197 [50] Internet Assigned Number Authority (IANA), "Address 4198 Literal Tags", 2007, 4199 . 4201 [51] RFC Editor, "RFC Errata - RFC 5321", 2019, 4202 . Captured 4203 2019-11-19 4205 [52] IANA, "SMTP Service Extensions", 2021, 4206 . Notes in draft: RFC 4208 Editor: Please adjust date field to reflect whatever you 4209 want for a registry that is updated periodically. IANA: 4210 Please determine if the above URL is a sufficiently stable 4211 reference and adjust as appropriate if it is not. 4213 Appendix A. TCP Transport Service 4215 The TCP connection supports the transmission of 8-bit bytes. The 4216 SMTP data is 7-bit ASCII characters. Each character is transmitted 4217 as an 8-bit byte with the high-order bit cleared to zero. Service 4218 extensions may modify this rule to permit transmission of full 8-bit 4219 data bytes as part of the message body, or, if specifically designed 4220 to do so, in SMTP commands or responses. 4222 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4224 Some systems use an RFC 822 header section (only) in a mail 4225 submission protocol, or otherwise generate SMTP commands from RFC 822 4226 header fields when such a message is handed to an MTA from a UA. 4227 While the MTA-UA protocol is a private matter, not covered by any 4228 Internet Standard, there are problems with this approach. For 4229 example, there have been repeated problems with proper handling of 4230 "bcc" copies and redistribution lists when information that 4231 conceptually belongs to the mail envelope is not separated early in 4232 processing from header field information (and kept separate). 4234 It is recommended that the UA provide its initial ("submission 4235 client") MTA with an envelope separate from the message itself. 4236 However, if the envelope is not supplied, SMTP commands SHOULD be 4237 generated as follows: 4239 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4240 be copied to a RCPT command (generating multiple message copies 4241 if that is required for queuing or delivery). This includes any 4242 addresses listed in a RFC 822 "group". Any BCC header fields 4243 SHOULD then be removed from the header section. Once this 4244 process is completed, the remaining header fields SHOULD be 4245 checked to verify that at least one TO, CC, or BCC header field 4246 remains. If none do, then a BCC header field with no additional 4247 information SHOULD be inserted as specified in [12]. 4249 2. The return address in the MAIL command SHOULD, if possible, be 4250 derived from the system's identity for the submitting (local) 4251 user, and the "From:" header field otherwise. If there is a 4252 system identity available, it SHOULD also be copied to the Sender 4253 header field if it is different from the address in the From 4254 header field. (Any Sender header field that was already there 4255 SHOULD be removed.) Systems may provide a way for submitters to 4256 override the envelope return address, but may want to restrict 4257 its use to privileged users. This will not prevent mail forgery, 4258 but may lessen its incidence; see Section 7.1. 4260 When an MTA is being used in this way, it bears responsibility for 4261 ensuring that the message being transmitted is valid. The mechanisms 4262 for checking that validity, and for handling (or returning) messages 4263 that are not valid at the time of arrival, are part of the MUA-MTA 4264 interface and not covered by this specification. 4266 A submission protocol based on Standard RFC 822 information alone 4267 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4268 system into an SMTP environment. Additional information to construct 4269 an envelope must come from some source in the other environment, 4270 whether supplemental header fields or the foreign system's envelope. 4272 Attempts to gateway messages using only their header "To" and "Cc" 4273 fields have repeatedly caused mail loops and other behavior adverse 4274 to the proper functioning of the Internet mail environment. These 4275 problems have been especially common when the message originates from 4276 an Internet mailing list and is distributed into the foreign 4277 environment using envelope information. When these messages are then 4278 processed by a header-section-only remailer, loops back to the 4279 Internet environment (and the mailing list) are almost inevitable. 4281 Appendix C. Placeholder (formerly Source Routes) 4283 // This entire section has been removed, with some material moved 4284 // into Appendix F.2. This comment is retained as a temporary 4285 // placeholder because the WG, the Ticket list, and various email 4286 // threads refer to Appendix letters and it would not be good to 4287 // create confusion about that while rfc5321bis is under development. 4289 Appendix D. Scenarios 4291 This section presents complete scenarios of several types of SMTP 4292 sessions. In the examples, "C:" indicates what is said by the SMTP 4293 client, and "S:" indicates what is said by the SMTP server. 4295 D.1. A Typical SMTP Transaction Scenario 4297 This SMTP example shows mail sent by Smith at host bar.com, and to 4298 Jones, Green, and Brown at host foo.com. Here we assume that host 4299 bar.com contacts host foo.com directly. The mail is accepted for 4300 Jones and Brown. Green does not have a mailbox at host foo.com. 4302 S: 220 foo.com Simple Mail Transfer Service Ready 4303 C: EHLO bar.com 4304 S: 250-foo.com greets bar.com 4305 S: 250-8BITMIME 4306 S: 250-SIZE 4307 S: 250-DSN 4308 S: 250 HELP 4309 C: MAIL FROM: 4310 S: 250 OK 4311 C: RCPT TO: 4312 S: 250 OK 4313 C: RCPT TO: 4314 S: 550 No such user here 4315 C: RCPT TO: 4316 S: 250 OK 4317 C: DATA 4318 S: 354 Start mail input; end with . 4319 C: Blah blah blah... 4320 C: ...etc. etc. etc. 4321 C: . 4322 S: 250 OK 4323 C: QUIT 4324 S: 221 foo.com Service closing transmission channel 4326 D.2. Aborted SMTP Transaction Scenario 4327 S: 220 foo.com Simple Mail Transfer Service Ready 4328 C: EHLO bar.com 4329 S: 250-foo.com greets bar.com 4330 S: 250-8BITMIME 4331 S: 250-SIZE 4332 S: 250-DSN 4333 S: 250 HELP 4334 C: MAIL FROM: 4335 S: 250 OK 4336 C: RCPT TO: 4337 S: 250 OK 4338 C: RCPT TO: 4339 S: 550 No such user here 4340 C: RSET 4341 S: 250 OK 4342 C: QUIT 4343 S: 221 foo.com Service closing transmission channel 4345 D.3. Relayed Mail Scenario 4347 Step 1 -- Source Host to Relay Host 4348 The source host performs a DNS lookup on XYZ.COM (the destination 4349 address) and finds DNS MX records specifying xyz.com as the best 4350 preference and foo.com as a lower preference. It attempts to open a 4351 connection to xyz.com and fails. It then opens a connection to 4352 foo.com, with the following dialogue: 4354 S: 220 foo.com Simple Mail Transfer Service Ready 4355 C: EHLO bar.com 4356 S: 250-foo.com greets bar.com 4357 S: 250-8BITMIME 4358 S: 250-SIZE 4359 S: 250-DSN 4360 S: 250 HELP 4361 C: MAIL FROM: 4362 S: 250 OK 4363 C: RCPT TO: 4364 S: 250 OK 4365 C: DATA 4366 S: 354 Start mail input; end with . 4367 C: Date: Thu, 21 May 1998 05:33:29 -0700 4368 C: From: John Q. Public 4369 C: Subject: The Next Meeting of the Board 4370 C: To: Jones@xyz.com 4371 C: 4372 C: Bill: 4373 C: The next meeting of the board of directors will be 4374 C: on Tuesday. 4375 C: John. 4376 C: . 4377 S: 250 OK 4378 C: QUIT 4379 S: 221 foo.com Service closing transmission channel 4381 Step 2 -- Relay Host to Destination Host 4382 foo.com, having received the message, now does a DNS lookup on 4383 xyz.com. It finds the same set of MX records, but cannot use the one 4384 that points to itself (or to any other host as a worse preference). 4385 It tries to open a connection to xyz.com itself and succeeds. Then 4386 we have: 4388 S: 220 xyz.com Simple Mail Transfer Service Ready 4389 C: EHLO foo.com 4390 S: 250 xyz.com is on the air 4391 C: MAIL FROM: 4392 S: 250 OK 4393 C: RCPT TO: 4394 S: 250 OK 4395 C: DATA 4396 S: 354 Start mail input; end with . 4397 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4398 C: 05:33:29 -0700 4399 C: Date: Thu, 21 May 1998 05:33:29 -0700 4400 C: From: John Q. Public 4401 C: Subject: The Next Meeting of the Board 4402 C: To: Jones@xyz.com 4403 C: 4404 C: Bill: 4405 C: The next meeting of the board of directors will be 4406 C: on Tuesday. 4407 C: John. 4408 C: . 4409 S: 250 OK 4410 C: QUIT 4411 S: 221 xyz.com Service closing transmission channel 4413 D.4. Verifying and Sending Scenario 4414 S: 220 foo.com Simple Mail Transfer Service Ready 4415 C: EHLO bar.com 4416 S: 250-foo.com greets bar.com 4417 S: 250-8BITMIME 4418 S: 250-SIZE 4419 S: 250-DSN 4420 S: 250-VRFY 4421 S: 250 HELP 4422 C: VRFY Crispin 4423 S: 250 Mark Crispin 4424 C: MAIL FROM: 4425 S: 250 OK 4426 C: RCPT TO: 4427 S: 250 OK 4428 C: DATA 4429 S: 354 Start mail input; end with . 4430 C: Blah blah blah... 4431 C: ...etc. etc. etc. 4432 C: . 4433 S: 250 OK 4434 C: QUIT 4435 S: 221 foo.com Service closing transmission channel 4437 Appendix E. Other Gateway Issues 4439 In general, gateways between the Internet and other mail systems 4440 SHOULD attempt to preserve any layering semantics across the 4441 boundaries between the two mail systems involved. Gateway- 4442 translation approaches that attempt to take shortcuts by mapping 4443 (such as mapping envelope information from one system to the message 4444 header section or body of another) have generally proven to be 4445 inadequate in important ways. Systems translating between 4446 environments that do not support both envelopes and a header section 4447 and Internet mail must be written with the understanding that some 4448 information loss is almost inevitable. 4450 Appendix F. Deprecated Features of RFC 821 4452 A few features of RFC 821 have proven to be problematic and SHOULD 4453 NOT be used in Internet mail. Some of these features were deprecated 4454 in RFC 2821 in 2001; source routing and two-digit years in dates were 4455 deprecated even earlier, by RFC 1123 in 1989. Of the domain literal 4456 forms, RFC 1123 required support only for the dotted decimal form. 4457 With the possible exception of old, hardware-embedded, applications, 4458 there is no longer any excuse for these features to appear on the 4459 contemporary Internet. 4461 F.1. TURN 4463 This command, described in RFC 821, raises important security issues 4464 since, in the absence of strong authentication of the host requesting 4465 that the client and server switch roles, it can easily be used to 4466 divert mail from its correct destination. Its use is deprecated; 4467 SMTP systems SHOULD NOT use it unless the server can authenticate the 4468 client. 4470 F.2. Source Routing 4472 RFC 821 utilized the concept of explicit source routing to get mail 4473 from one host to another via a series of relays. Source routes could 4474 appear in either the or to show the 4475 hosts through which mail would be routed to reach the destination. 4476 The requirement to utilize source routes in regular mail traffic was 4477 eliminated by the introduction of the domain name system "MX" record 4478 by RFC 974 in early 1986 and the last significant justification for 4479 them was eliminated by the introduction, in RFC 1123, of a clear 4480 requirement that addresses following an "@" must all be fully- 4481 qualified domain names. Issues involving local aliases for mailboxes 4482 were addressed by the introduction of a separate specification for 4483 mail submission [41]. Consequently, there are no remaining 4484 justifications for the use of source routes other than support for 4485 very old SMTP clients. Even use in mail system debugging is unlikely 4486 to work because almost all contemporary systems either ignore or 4487 reject them. 4489 Historically, for relay purposes, the forward-path may have been a 4490 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4491 THREE MUST be fully-qualified domain names. This form was used to 4492 emphasize the distinction between an address and a route. The 4493 mailbox (here, JOE@THREE) is an absolute address, and the route is 4494 information about how to get there. The two concepts should not be 4495 confused. 4497 SMTP servers SHOULD continue to accept source route syntax as 4498 specified in this appendix. If they do so, they SHOULD ignore the 4499 routes and utilize only the target domain in the address. If they do 4500 utilize the source route, the message MUST be sent to the first 4501 domain shown in the address. In particular, a server MUST NOT guess 4502 at shortcuts within the source route. SMTP clients SHOULD NOT 4503 attempt to utilize explicit source routing. 4505 If source routes appear in mail received by an SMTP server contrary 4506 to the requirements and recommendations in this specification, RFC 4507 821 and the text below should be consulted for the mechanisms for 4508 constructing and updating the forward-path. A server that is reached 4509 by means of a source route (e.g., its domain name appears first in 4510 the list in the forward-path) MUST remove its domain name from any 4511 forward-paths in which that domain name appears before forwarding the 4512 message and MAY remove all other source routing information. Any 4513 source route information in the reverse-path SHOULD be removed by 4514 servers conforming to this specification. 4516 The following information is provided for historical information 4517 only, so that the source route syntax and application can be 4518 understood if needed. 4520 Syntax: 4521 The original form of the production in Section 4.1.2 was: 4523 Path = "<" [ A-d-l ":" ] Mailbox ">" 4525 A-d-l = At-domain *( "," At-domain ) 4527 At-domain = "@" Domain 4529 For example, suppose that a delivery service notification must be 4530 sent for a message that arrived with: 4531 MAIL FROM:<@a.example,@b.example:user@d.example> 4532 The notification message MUST be sent using: 4533 RCPT TO: 4535 F.3. HELO 4537 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4538 than HELO when the server will accept the former. Servers MUST 4539 continue to accept and process HELO in order to support older 4540 clients. 4542 F.4. #-literals 4544 RFC 821 provided for specifying an Internet address as a decimal 4545 integer host number prefixed by a pound sign, "#". In practice, that 4546 form has been obsolete since the introduction of TCP/IP. It is 4547 deprecated and MUST NOT be used. 4549 F.5. Dates and Years 4551 When dates are inserted into messages by SMTP clients or servers 4552 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4553 digit years are deprecated; three-digit years were never permitted in 4554 the Internet mail system. 4556 F.6. Sending versus Mailing 4558 In addition to specifying a mechanism for delivering messages to 4559 user's mailboxes, RFC 821 provided additional, optional, commands to 4560 deliver messages directly to the user's terminal screen. These 4561 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4562 workstation technology and the introduction of other protocols may 4563 have rendered them obsolete even where they are implemented. 4565 Clients SHOULD NOT use SEND, SAML, or SOML commands. If a server 4566 implements them, the implementation model specified in RFC 821 [3] 4567 MUST be used and the command names MUST be published in the response 4568 to the EHLO command. 4570 Appendix G. Other Outstanding Issues 4572 [[RFC Editor: Please remove this section before publication.]] 4574 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4575 that led to a broad discussion of ways in which existing practice had 4576 diverged from the specifications and recommendations of RFC 5321 in 4577 the more than eleven years since it was published (some of those 4578 issues probably affect the boundary between RFC 5321 and 5322 and 4579 hence the latter as well). In most cases, those divergences call for 4580 revision of the Technical Specification to match the practice, 4581 clarification of the specification text in other ways, or a more 4582 comprehensive explanation of why the practices recommended by the 4583 specification should really be followed. 4585 Those discussions raised two other issues, which were that 4587 * The publication of the Submission Server specification of RFC 6409 4588 in November 2011 may not have been fully reflected in RFC 5321 4589 (despite the even earlier publication of RFC 4409) and 4591 * There may be inconsistencies between the July 2009 Internet Mail 4592 Architecture description of RFC 5598 and the model described in 4593 RFC 5321. The issue called out in Appendix G.3 below may be an 4594 example of one of those inconsistencies. 4596 Those discrepancies should be identified and discussed and decisions 4597 made to fix them (and where) or to ignore them and let them continue. 4599 There has also been discussion on the mailing list, perhaps amounting 4600 to very rough consensus, that any revision of RFC 5321 and/or 5322 4601 should be accompanied by a separate Applicability Statement document 4602 that would make recommendations about applicability or best practices 4603 in particular areas rather than trying to get everything into the two 4604 technical specifications. This appendix does not attempt to identify 4605 which issues should get which treatment. 4607 This work is now (starting in the last half of 2020) being considered 4608 in the EMAILCORE WG. This appendix will act as a temporary record of 4609 issues that should be discussed and decided upon before a revised 4610 SMTP specification (or a related Applicability Statement) is 4611 published, issues that have not been reflected in errata (see 4612 Appendix H.1 below for those covered by errata). 4614 Ticket numbers listed below reference the list in 4615 https://trac.ietf.org/trac/emailcore/report/1 . 4617 G.1. IP Address literals 4619 The specification is unclear about whether IP address literals, 4620 particularly IP address literals used as arguments to the EHLO 4621 command, are required to be accepted or whether they are allowed to 4622 be rejected as part of the general "operational necessity" exception. 4623 Some have suggested that rejection of them is so common as an anti- 4624 spam measure that the use of such literals should be deprecated 4625 entirely in the specification, others that the are still useful and 4626 used and/or that, whatever is said about IP address literals within 4627 an SMTP session (e.g., in MAIL or RCPT commands), they should 4628 continue to be allowed (and required) in EHLO. 4629 Ticket #1 (issue for A/S). 4631 G.2. Repeated Use of EHLO (closed) 4633 While the specification says that an SMTP client's sending EHLO again 4634 after it has been issued (starting an SMTP session and treats it as 4635 if RSET had been sent (closing the session) followed by EHLO, there 4636 are apparently applications, at least some of them involving setting 4637 up of secure connections, in which the second EHLO is required and 4638 does not imply RSET. Does the specification need to be adjusted to 4639 reflect or call out those cases? 4640 After extended discussion in October 2020, it appears that the 4641 easiest fix to these problems is to clarify the conditions for 4642 termination of a mail transaction in Section 3.3 and to clearly 4643 specify the effect of a second (or subsequent) EHLO command in 4644 Section 4.1.4. 4645 See also Appendix G.7.4. 4646 Ticket #2. (closed - Both changes have been made in draft-ietf- 4647 emailcore-rfc5321bis-01). 4649 G.3. Meaning of "MTA" and Related Terminology 4651 A terminology issue has come up about what the term "MTA" actually 4652 refers to, a question that became at least slightly more complicated 4653 when we formalized RFC 6409 Submission Servers. Does the document 4654 need to be adjusted to be more clear about this topic? Note that the 4655 answer may interact with the question asked in Section 2 above. 4656 Possibly along the same lines, RFC 2821 changed the RFC 821 4657 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4658 and "SMTP server" respectively. As things have evolved, it is 4659 possible that newer terminology is a source of confusion and that the 4660 terminology should be changed back, something that also needs 4661 discussion. 4662 Ticket #3. 4664 G.4. Originator, or Originating System, Authentication 4666 Should RFC 5321bis address authentication and related issues or 4667 should Section 3.4.2 or other text be reshaped (in addition to or 4668 instead of the comment on that section) to lay a better foundation 4669 for such work, either in the context of mailing lists or more 4670 generally? 4671 This may interact with Erratum 4055 and Ticket #30 below. 4673 G.5. Remove or deprecate the work-around from code 552 to 452 (closed) 4675 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4676 and/or be inconsistent with current practice. Should it be removed 4677 and/or explicitly deprecated? 4678 Ticket #5 (fixed and closed). 4680 SHOULD requirement removed. 4682 G.6. Clarify where the protocol stands with respect to submission and 4683 TLS issues 4685 1. submission on port 587 4687 2. submission on port 465 4688 3. TLS relay on a port different from 25 (whenever) 4690 4. Recommendations about general use of transport layer (hop by hop) 4691 security, particularly encryption including consideration of RFC 4692 8314. 4694 G.7. Probably-substantive Discussion Topics Identified in Other Ways 4696 The following issues were identified as a group in the opening Note 4697 but called out specifically only in embedded CREF comments in 4698 versions of this draft prior to the first EMAILCORE version. 4700 G.7.1. Issues with 521, 554, and 556 codes (closed) 4702 See new Section 4.2.4.2. More text may be needed, there or 4703 elsewhere, about choices of codes in response to initial opening and 4704 to EHLO, especially to deal with selective policy rejections. In 4705 particular, should we more strongly discourage the use of 554 on 4706 initial opening. And should we make up a 421 code (or a new 4yz 4707 code, perhaps 454) code for situations where the server is 4708 temporarily out of service? 4709 Ticket #6 (closed). 4711 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 4713 CREF comment in Section 2, CREF comment in Section 2.3.10, and 4714 comments in the introductory portion of Appendix G. 4716 G.7.3. Resolvable FQDNs and private domain names 4718 Multiple CREF comments in Section 2.3.5 4719 Tickets #9 (definition of domain name), #10 (meaning of "resolvable 4720 domain name"), and #41 (closed -- no change 2021-04-05). 4722 G.7.4. Possible clarification about mail transactions and transaction 4723 state 4725 CREF comment in Section 3.3 and also reference in Section 4.1.4 4726 Ticket #11. 4728 // See correspondence on this ticket 2021-07-06 through 2021-07-09. 4730 G.7.5. Issues with mailing lists, aliases, and forwarding 4732 CREF comment in Section 3.4.2. May also want to note forwarding as 4733 an email address portability issue. Note that, if changes are made 4734 in this area, they should be kept consistent with the description and 4735 discussion of the 251 and 551 in Section 4.2 and Section 3.5 as well 4736 as Section 3.4.1 to avoid introducing inconsistencies. In addition, 4737 there are some terminology issues about the use of the term "lists", 4738 identified in erratum 1820, that should be reviewed after any more 4739 substantive changes are made to the relevant sections. 4740 Ticket #12 and Ticket #34 (Ticket #34/ erratum 1820 resolved in -06 4741 and closed). 4743 G.7.6. Requirements for domain name and/or IP address in EHLO 4745 Text in Section 4.1.4; change made in -05. 4746 Ticket #19. 4748 G.7.7. Does the 'first digit only' and/or non-listed reply code text 4749 need clarification? (closed) 4751 Resolved. Text in Section 4.2 changed 2021-02-08 and CREF comment in 4752 Section 4.3.1 removed. 4754 Perhaps unresolved -- ongoing discussion on mailing list after IETF 4755 110. 4756 Ticket #13 (fixed and closed). 4758 G.7.8. Size limits (closed) 4760 Once a decision is made about line length rules for RFC 5322bis, 4761 review the size limit discussions in this document, particularly the 4762 CREF comment (Note in Draft) at the end of the introductory material 4763 to Section 4.5.3 to be sure this document says what we want it to 4764 say. (See the additional question about minimum quantities, etc., in 4765 Appendix G.7.19.) 4766 Ticket #14 (closed - no action) and maybe Ticket #38 (to A/S). 4768 G.7.9. Discussion of 'blind' copies and RCPT 4770 CREF comment in Section 7.2. May also need to discussion whether 4771 that terminology is politically incorrect and suggest a replacement. 4772 Ticket #15. 4774 G.7.10. Further clarifications needed to source routes? 4776 The current text largely deprecates the use of source routes but 4777 suggests that servers continue to support them. Is additional work 4778 needed in this area? See CREF comment in Appendix F.2 4779 Ticket #17. 4781 G.7.11. Should 1yz Be Revisited? (closed) 4783 RFC 5321 depreciated the "positive preliminary reply" response code 4784 category with first digit "1", so that the first digit of valid SMTP 4785 response codes must be 2, 3, 4, or 5. It has been suggested (see 4786 mail from Hector Santos with Subject "SMTP Reply code 1yz Positive 4787 Preliminary reply", March 5, 2020 12:56 -0500, on the SMTP list) that 4788 these codes should be reinstated to deal with some situations that 4789 became more plausible after 5321 was published. Do we need to take 4790 this back up? 4791 Ticket #18 (no, closed). 4793 G.7.12. Review Timeout Specifications 4795 RFC 5321 (and its predecessors going back to 821) specify minimum 4796 periods for client and server to wait before timing out. Are those 4797 intervals still appropriate in a world of faster processors and 4798 faster networks? Should they be updated and revised? Or should more 4799 qualifying language be added? 4800 Ticket #16. 4802 G.7.13. Possible SEND, SAML, SOML Loose End (closed) 4804 Per discussion (and Ticket #20), the text about SEND, SAML, and SOML 4805 has been removed from the main body of the document so that the only 4806 discussion of them now appears in Appendix F.6. Per the editor's 4807 note in that appendix, is any further discussion needed? 4808 Ticket #20 (closed) 4810 G.7.14. Abstract Update (closed) 4812 Does the Abstract need to be modified in the light of RFC 6409 or 4813 other changes? 4814 Ticket #52 (changes made; closed) 4816 G.7.15. Informative References to MIME and/or Message Submission 4817 (closed) 4819 Should RFC 2045 (MIME) and/or RFC 6409 (Message Submission) be 4820 referenced at the end of Section 1.2? 4821 Ticket #53 (more general reference to the A/S, closed). 4823 G.7.16. Mail Transaction Discussion 4825 Does the discussion of mail transactions need more work (see CREF in 4826 Section 3.3.)? 4828 G.7.17. Hop by hop Authentication and/or Encryption (closed) 4830 Should this document discuss hop-by-hop authentication or, for that 4831 matter, encryption? (See CREF in Section 2.) 4832 Propose "No, it shouldn't" (20211101 conversation with Todd.) 4833 Ticket #50 (work with in A/S. Closed). 4835 G.7.18. More Text About 554 Given 521, etc. 4837 Does reply code 554 need additional or different explanation in the 4838 light of the addition of the new 521 code and/or the new (in 5321bis 4839 Section 4.2.4.2? (See CREF in Section 4.2.3.) 4841 G.7.19. Minimum Lengths and Quantities 4843 Are the minimum lengths and quantities specified in Section 4.5.3 4844 still appropriate or do they need adjusting? (See CREF at the 4845 beginning of that section.) Also note potential interaction with the 4846 proposed LIMITS SMTP extension (draft-freed-smtp-limits) which may 4847 make this question OBE. 4849 G.8. Enhanced Reply Codes and DSNs 4851 Enhanced Mail System Status Codes (RFC 3463) [7] were added to SMTP 4852 before RFC 5321 was published and are now, together with a 4853 corresponding registry [43], widely deployed and in extensive use in 4854 the network. Similar, the structure and extensions options for 4855 Delivery Status Notifications [35] is implemented, deployed, and in 4856 wide use. Is it time to fold all or part of those mature 4857 specifications into the SMTP spec or at least to mention and 4858 normatively reference them? And, as an aside, do those specs need 4859 work or, if they are kept separate, is it time to move them to 4860 Internet Standard? 4862 At least one of the current references to RFC 3463 indicates that it 4863 SHOULD be used. That presumably makes the reference normative 4864 because one needs that specification to know what the present 4865 document requires. It has been moved in the -03 version of this 4866 draft, but, unless it is move to Internet Standard, it will require 4867 downref treatment. 4869 G.9. Revisiting Quoted Strings 4871 Recent discussions both in and out of the IETF have highlighted 4872 instances of non-compliance with the specification of a Local-part 4873 consisting of a Quoted-string, whether any content of QcontentSMTP 4874 that actually requires special treatment consists of qtextSMTP, 4875 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4876 above) ends with a few paragraphs of warnings (essentially a partial 4877 applicability statement), the first of which cautions against 4878 cleverness with either Quoted-string or case sensitivity as a threat 4879 to interoperability. 4881 The Quoted-string portion of that discussion has apparently been 4882 widely not read or ignored. Do we need to do something else? If we 4883 do an Applicability Statement, would it be useful to either reference 4884 the discussion in this document from there or to move the discussion 4885 there and reference it (normatively?) from here? 4887 There has been a separate discussion of empty quoted strings in 4888 addresses, i.e., whether the production should be 4889 required to included at least one non-whitespace character. It is 4890 separate from this issue but would be further impacted or distorted 4891 from the considerations identified in this Section. 4893 Text modified in -07. 4894 Ticket #21. May also interact with Ticket #35. 4896 G.10. Internationalization 4898 RFC 5321 came long before work on internationalization of email 4899 addresses and headers (other than by use of encoded words in MINE) 4900 and specifically before the work of the EAI WG leading to the 4901 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4902 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4903 define mailboxes ...") is an extremely strong prohibition against the 4904 use of non-ASCII characters in SMTP commands and the requirements 4905 about message content in Section 2.3.1 an equally strong one for 4906 content. Would it be appropriate to add something like "in the 4907 absence of relevant extensions" there? Also, given [mis]behavior 4908 seen in the wild, does that paragraph (or an A/S) need an explicit 4909 caution about SMTP servers or clients assuming they can apply the 4910 popular web convention of using %NN sequences as a way to encode non- 4911 ASCII characters ( in RFC 3986) and assuming some later 4912 system will interpret it as they expect? Would it be appropriate to 4913 add an Internationalization Considerations section to the body of 4914 this document if only for the purpose of pointing people elsewhere? 4915 More broadly, while the EAI WG's extensions for non-ASCII headers and 4916 addresses are explicitly out of scope for the EMAILCORE WG (at least 4917 for 5321bis (and 5322bis), those documents make assumptions and 4918 interpretations of the core documents. Are there areas in which 4919 5321bis could and should be clarified to lay a more solid foundation 4920 for the EAI/SMTPUTF8 work and, if so, what are they? 4922 G.11. SMTP Clients, Servers, Senders, and Receivers 4924 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4925 2821 (and hence in 5321), we switched that to "client" and "server" 4926 (See the discussion in Section 1.2). In part because a relay is a 4927 server and then a client (in some recent practice, even interleaving 4928 the two functions by opening the connection to the next host in line 4929 and sending commands before the incoming transaction is complete), 4930 RFC 5321 continues to use the original terminology in some places. 4931 Should we revisit that usage, possibly even returning to consistent 4932 use of the original terminology? 4934 G.12. Extension Keywords Starting in 'X-' (closed) 4936 Section 2.2.2 contains a discussion of SMTP keywords starting in "X". 4937 Given general experience with such things and RFC 6648, is there any 4938 reason to not deprecate that practice entirely and remove that text? 4939 If we do so, should the former Section 4.1.5 be dropped or rewritten 4940 to make clear this is an obsolete practice? 4941 4.1.5 eliminated in rfc5321bis-06. 4942 Ticket #42 (resolved with -06 and closed). 4944 G.13. Deprecating HELO (closed) 4946 RFC 5321 (and 2821 before it) very carefully circle around the status 4947 of HELO, even recommending its use as a fallback when EHLO is sent 4948 and a "command not recognized" response is received. We are just a 4949 few months short of 20 years; is it time to deprecate the thing and 4950 clean out some or all of that text? And, given a recent (4Q2020) 4951 discussion on the EMAILCORE list, should EHLO be explicitly bound to 4952 SMTP over TCP with the older transports allowed only with HELO? 4953 While those questions may seem independent, separating them is fairly 4954 hard given the way the text is now constructed. 4956 Resolved 2021-01-19: No change 4957 Ticket #43 (closed). 4959 G.14. The FOR Clause in Trace Fields: Semantics, Security 4960 Considerations, and Other Issues 4962 The FOR clause in time-stamp ("Received:") fields is seriously under- 4963 defined. It is optional, the syntax is clear, but its semantics and 4964 use, while perhaps obvious from content and the application of common 4965 sense, have never been defined ("never" going back to 821). Do we 4966 want to better define it? Is there any chance that a definition 4967 would invalid existing, conforming and sensible, implementations? If 4968 we do want to define semantics, draft text and advice as to where it 4969 should go are invited. 4971 (Paragraph added 2021-08-18) 4972 In particular, recentdiscussions point strongly to the need for a 4973 statement to the effect that the value of the for clause must contain 4974 one of the addresses that caused the message to be routed to the 4975 recipient of this message copy (thanks Ned), that no mare than one 4976 address can appear, and that showing one address when there are 4977 multiple RCPT commands may be a security and/or privacy issue (thanks 4978 Ned and Viktor and see ). More detailed or specific 4980 guidance, including case analysis, are probably material for the A/s, 4981 but that is obviously up to the WG. 4983 Note the existing discussions in Section 7.2 and Section 7.6 as they 4984 may need adjustment, or at least cross-references, especially if FOR 4985 is more precisely defined. 4987 There is probably an error in Section 7.6. Its last sentence implies 4988 a possible interaction between messages with multiple recipients and 4989 the FOR clause of trace fields. However, because the syntax of the 4990 FOR clause only allows one Mailbox (or Path), it isn't clear if that 4991 statement is meaningful. Should it be revised to discuss other 4992 situations in which including FOR might not be desirable from a 4993 security or privacy standpoint? (See above -- this paragraph 4994 deliberately not changed in -04). 4995 Ticket #55 4997 G.15. Resistance to Attacks and Operational Necessity (closed) 4999 Section 7.8 is often cited as allowing an exception to the rules of 5000 the specification for reasons of operational necessity, not just 5001 attack resistance. I (JcK) believe the broader interpretation was 5002 intended by YAM (the section was new in RFC 5321). Recommendation: 5003 change the title to explicitly include "Local Operational 5004 Requirements" and add text to indicate that attack resistance is not 5005 the only possible source of such requirements. 5006 Ticket #48 (done, closed) 5008 G.16. Mandatory 8BITMIME 5010 There was extensive discussion on the mailing list in October 2021 5011 about messages with and without 8-bit (i.e., octets with the leading 5012 on) content and a tentative conclusion that support for 8BITMIME 5013 should be required. If that is the WG's conclusion, we need to 5014 figure out what to say and where to say it. 5016 Appendix H. RFC 5321 Errata Summary and Tentative Change Log 5018 [[RFC Editor: Please remove this section before publication.]] 5020 H.1. RFC 5321 Errata Summary 5022 This document addresses the following errata filed against RFC 5321 5023 since its publication in October 2008 [51]. As with the previous 5024 appendix, ticket numbers included below reference 5025 https://trac.ietf.org/trac/emailcore/report/1 . 5026 // [[Note in Draft: Unless marked "closed", items with comments below 5027 // have not yet been resolved as errata.]] 5029 1683 ABNF error. (closed) Section 4.4 5030 Ticket #23 (fixed, closed). 5032 4198 Description error. (closed) Section 4.2. 5033 RESOLVED 2020-12-14, ticket #24 (closed). 5035 2578 Syntax description error. (closed) Section 4.1.2 5036 Ticket #25 (fixed, closed) 5038 1543 Wrong code in description (closed) Section 3.8 5039 Ticket #26 (fixed, closed) 5041 4315 ABNF - IPv6 Section 4.1.3 (closed). 5042 // [5321bis]The IPv6 syntax has been adjusted since 5321 was 5043 // published (the erratum mentions RFC 5952, but RFC 6874 and 5044 draft- 5045 // carpenter-6man-rfc6874bis should also be considered). See the 5046 // rewritten form and the comment in the section cited in the 5047 // previous sentence, at least for the RFC 5952 issues. The 5048 editor 5049 // awaits instructions. See https://www.rfc-editor.org/errata/ 5050 // eid4315 5051 Ticket #27 (closed 2021-01-19). 5053 5414 ABNF for Quoted-string (closed) Section 4.1.2 5054 Ticket #22 (fixed, closed). 5056 1851 Location of text on unexpected close Section 4.1.1.5 (closed). 5057 Text moved per email 2020-12-31. 5058 Ticket #28 (fixed, closed). 5060 3447 Use of normative language (e.g., more "MUST"s), possible 5061 confusion in some sections Section 4.4. 5062 Ticket #7 5064 // [5321bis]As Barry notes in his verifier comments on the erratum 5065 // (see https://www.rfc-editor.org/errata/eid3447), the comments 5066 and 5067 // suggestions here raise a number of interesting (and difficult) 5068 // issues. One of the issues is that the core of RFCs 5321 (and 5069 // 2821) is text carried over from Jon Postel's RFC 821, a 5070 document 5071 // that was not only written in a different style than the IETF 5072 uses 5073 // today but that was written at a time when no one had dreamt of 5074 RFC 5075 // 2119 or even the IETF itself. It appears to me that trying to 5076 // patch that style might easily result in a document that is 5077 harder 5078 // to read as well as being error prone. If we want to get the 5079 // document entirely into contemporary style, we really should 5080 bite 5081 // the bullet and do a complete rewrite. To respond to a 5082 different 5083 // point in Barry's discussion, I think an explicit statement that 5084 // 5321/5322 and their predecessors differ in places and why would 5085 be 5086 // helpful. Text, and suggestions about where to put it, are 5087 // solicited. A list of differences might be a good idea too, but 5088 // getting it right might be more work than there is available 5089 energy 5090 // to do correctly. 5092 5711 Missing leading spaces in example Appendix D.3 (closed). 5094 // [5321bis]Well, this is interesting because the XML is correct 5095 and 5096 // the spaces are there, embedded in artwork. So either the 5097 XML2RFC 5098 // processor at the time took those leading spaces out or the RFC 5099 // Editor improved on the document and the change was not caught 5100 in 5101 // AUTH48, perhaps because rfcdiff ignores white space. We just 5102 need 5103 // to watch for future iterations. 5105 As of 2021-03-15, both the txt and html-ized versions of draft- 5106 ietf-emailcore-rfc5321bis-02 were showing identical output for 5107 both parts of the example, so the problem appears to be OBE at 5108 worst. 5109 Ticket #29 (closed 2021-03-16) 5111 4055 (closed) Erratum claims the the description of SPF and DKIM is 5112 wrong. It is not clear what 5321bis should really say about them, 5113 but the current text probably needs work (or dropping, which is 5114 what the proposed erratum suggests). 5115 Text changed; ticket should probably be closed after WG reviews 5116 -04. 5117 Ticket #30 (resolved and closed). 5119 // [5321bis]Note that rejected errata have _not_ been reviewed to see 5120 // if they contain anything useful that should be discussed again 5121 // with the possibility of rethinking and changing text. Volunteers 5122 // sought. 5124 H.2. Changes from RFC 5321 (published October 2008) to the initial 5125 (-00) version of this draft 5127 * Acknowledgments section (Section 9) trimmed back for new document. 5129 * Introductory paragraph to Appendix F extended to make it clear 5130 that these features were deprecated a long time ago and really 5131 should not be in use any more. 5133 * Adjusted some language to clarify that source routes really, 5134 really, should not be used or depended upon. 5136 * IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 5137 a note added. 5139 * Production index added as a first step in tying all productions to 5140 their sources. As part of the effort to make the document more 5141 easily navigable, table of contents entries have been created for 5142 the individual command descriptions. 5144 * Clarified the relationship between the SMTP "letters, digits, and 5145 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 5147 * Revised the reply code sections to add new 521 and 556 codes, 5148 clarify relationships, and be explicit about the requirement for 5149 clients to rely on first digits rather than the sequences in 5150 Section 4.3.2. 5152 * In conjunction with the above, explicitly obsolete RFCs 1846 and 5153 7504 (but that might not be right -- see email 2021-10-03. 5155 * Incorporated a correction reflecting Errata ID 2578. 5157 * Some small editorial changes made to eliminate redundant 5158 statements that were very close together. Other, equally small, 5159 editorial changes have been made to improve grammar or clarity. 5161 * A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 5162 Draft" have been added for the group to resolve. Other questions, 5163 especially those in the errata summary, are simply included in 5164 narrative comments in CREFs. 5166 * Checked and rationalized "response" (to a command) and "reply 5167 code" terminology. One can talk about a "999 response" but only a 5168 "999 reply code". There is no such thing as a "response code". 5170 * Added note about length limit on mailbox names ("email 5171 addresses"). 5173 * Added an "errata summary" subsection to this change log/ 5174 comparison to 5321 in this Appendix. The entire Appendix will, of 5175 course, disappear at the time of RFC publication unless someone 5176 wants to make a strong case for retaining it. 5178 * Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 5179 readers below the Abstract. 5181 * Temporarily added a "Note on Reading This Working Draft" after the 5182 Abstract. 5184 H.3. Changes Among Versions of Rfc5321bis 5186 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 5187 -01 5189 Substantively, these two versions differ only by suppression of the 5190 CREF and other discussion associated with the evolution from RFC 2821 5191 to RFC 5321. That change includes an update to the document's Note 5192 to Readers, the date, the file name, and the addition of this change 5193 log subsection. 5195 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 5197 * Minor clarifications to improve text, e.g., addition of NOOP to 5198 the list of non-mail transaction examples in Section 4.1.4. 5200 * Added topics exposed in the ietf-smtp list and the IETF list 5201 "dogfood" discussion during December 2019 and an index listing of 5202 substantive issues identified only in CREFs in the prior draft as 5203 a new Appendix G.. 5205 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 5207 * Added more text to Appendix G.7.1 to specifically call out the 5208 session-opening policy issues surrounding these codes. 5210 * Added discussion of "1yz" reinstatement in Appendix G.7.11. 5212 * Added discussion of timeouts in Appendix G.7.12. 5214 * Added subsection on Enhanced Status Codes and DSNs to the 5215 outstanding issues list Appendix G.8. 5217 * Replaced reference to RFC 1652 (8BITMIME) with the Internet 5218 Standard version, RFC 6152. 5220 * With help from cketti, clarified the ABNF productions whose 5221 terminals appear in other documents. 5223 * Added discussions of Quoted-string, Internationalization, and 5224 client-server versus sender-receiver terminology to Appendix G. 5226 * Added note to the Abstract. 5228 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) to draft- 5229 ietf-emailcore-rfc5321bis-00 5231 * Added a paragraph about non-null quoted strings to Appendix G.9. 5233 * Added an explicit pointer to email insecurity and TLS to 5234 Appendix G.6. Inspired by Ben Kaduk's comment on the WG Charter, 5235 2020-09-09. 5237 * Converted document from individual to emailcore WG effort. 5239 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 (2020-10-06) to 5240 -01 5242 * Editorial: Corrected "blackslash" to "backslash" 5244 * Rewrote the introduction to Appendix G slightly to reflect the 5245 creation of the EMAILCORE WG. 5247 * Applied fixes for repeated use of EHLO. See Appendix G.2. 5249 * Added two new questions, one about "X" extensions (Appendix G.12) 5250 and one about the status of HELO (Appendix G.13). 5252 * Removed mention of SEND, SAML, SOML from the main body of the text 5253 (Ticket #20). 5255 * Added a warning about side effects to Appendix G.7.5. 5257 * Added ticket numbers to descriptions of issues and changes, 5258 adjusted some text so relationships would be more clear, and added 5259 subsections to the Appendix G and H lists to pick up on tickets 5260 that were not easily identified in those sections of with the 5261 text. 5263 * Made several additions to the Index, including one to deal with 5264 SEND et al., as above. 5266 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 (2020-12-25) to 5267 -02 5269 * Corrected discussion mailing list to point to emailcore@ietf.org 5270 in the introductory note. 5272 * Added new subsection(s) to Appendix G to reflect newly discovered 5273 issues. 5275 * Changed "as discussed in" references in Section 4.5.5 per ticket 5276 #45. 5278 * Corrected a misleading use of the term "mailbox" in Section 3.3. 5280 * Changed descriptions of use of first digit in replies per ticket 5281 #13. See Appendix G.7.7. 5283 * Moved paragraph per ticket #28, erratum 1851. 5285 * Added more clarifying cross-references, clarified some CREFs, and 5286 cleaned out some of those that no longer seemed relevant. 5288 * Removed "updates 1123" is unnecessary and obsolete. 5290 * Updated several references. 5292 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 (2021-02-21) to 5293 -03 5295 * Editorial: Fixed some instances of constructions like "RCPT TO 5296 command". The name of the command is RCPT. Sloppy editing in 5297 2008. 5299 * Added text and cross-references to clarify the role of 452 and 552 5300 in "too many recipients" situations. 5302 * Added Appendix G.15 to discuss changes to better reflect 5303 "operational necessity" issue. 5305 * Added detail for erratum 5711, ticket #29. 5307 * Added new subsections of Appendix G.7 to keep some previously- 5308 unnoted CREF notes from getting lost. Also removed some CREFs 5309 that were notes on changes made before the WG was created or 5310 appeared to no longer have value and trimmed or rewrote some of 5311 the remaining ones. 5313 * More discussion of Ticket #13, See Appendix G.7.7. 5315 * Identified Ticket #41 as closed. See Appendix Appendix G.7.3; 5316 notes removed from Section 2.3.5. 5318 * "SHOULD" requirement for interpreting 552 "too many recipients" 5319 removed from Section 4.5.3.1.10, explanation added, and text 5320 cleaned up. Also removed the parenthetical historical notes on 5321 the return code definitions in Section 4.2. See Appendix G.5. 5322 (Ticket #5) 5324 * Modified Appendix G.8 to add a note about the normative status of 5325 RFC 3463 and moved that reference. 5327 * Several clarifications to initiation and termination of mail 5328 transactions in Section 4.1.4. 5330 * Several additional minor editorial improvements. 5332 * Note for drafts -03 and -04 only, modified somewhat for -05 but 5333 outdated from -06 forward: Some issues are still outstanding: 5334 Notes were posted to the list on 2021-07-09 about tickets #7 5335 (5322bis issue), #10 , #14 (closed), #20 (closed), #30 (closed), 5336 and #42 (closed). Even though some comments about them appeared 5337 in the subsequent day or so, there appears to have been 5338 insufficient time for discussions to stabilize sufficiently for 5339 changes to be included in this version of the I-D. 5341 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 (2021-07-10) to 5342 -04 5344 * Clarified that the "period" in . is really the ASCII 5345 one in Section 3.3. 5347 // Editor's note: change treated as Editorial without a ticket. 5348 If 5349 // there are objections, speak up. 5351 * Several other small editorial corrections. 5353 * Added several notes about the possible need to add text to reflect 5354 the presence of MSAs and to clarify whether MUAs send messages 5355 directly to MTAs or whether, in that case, the MUAs are just 5356 incorporating MSA functions. 5358 * Added new text to Appendix G.14 reflecting discussions of the 5359 Received...FOR issue. 5361 * Adjusted discussion of erratum 4315 (Ticket #27) to reflect more 5362 recent IPv6 syntax developments. 5364 * Adjusted discussion of the various "mail not accepted" codes, 5365 rewrote Section 4.2.4.2, annotated and inserted cross-references 5366 in relevant response code descriptions and (tentatively) 5367 identified this document as obsoleting RFC 7505. Editor's guess, 5368 reinforced by a brief conversation with John Levine (lead author 5369 of 7505), is that we should incorporate text as needed and 5370 obsolete it. The changes include replacing the reference to the 5371 "nullMX" I-D with RFC 7505, which I am appalled that neither I nor 5372 anyone else noticed earlier. Cf. Appendix G.7.1, Section 4.2.4.2, 5373 and Ticket #6. 5375 H.3.9. Changes from draft-ietf-emailcore-rfc5321bis-04 (2021-10-03) to 5376 -05 5378 * Took a first step toward rewriting and updating the introductory 5379 material. It is only a first step; suggestions welcome. 5381 * Minor editorial fixes. 5383 * Correct text about domain name checking in Section 4.1.4, probably 5384 fixing ticket #19. See CREF added there. 5386 * Added Appendix G.16 a placeholder for the 8BITMIME discussion and 5387 possible action. 5389 * Additional changes to the description and organization of trace 5390 field materials. Intended to resolve the 5321bis part of Ticket 5391 #7. 5393 * Remaining patch to SEND, etc., discussion in Appendix F.6 applied 5394 and CREF removed. 5396 * Removed discussion of "X-" and edited associated text. The fix 5397 may or may not be sufficient to resolve Ticket #42 (later closed). 5399 * Verified that the problems of getting four-level sections (e.g., 5400 "4.1.1.1" and other command-specific ones) into the table of 5401 contents and the index reflecting page numbers still exist and 5402 updated the introductory note. 5404 H.3.10. Changes from draft-ietf-emailcore-rfc5321bis-05 (2021-10-24) to 5405 -06 5407 * Finished making changes for "X-" and commands starting in "X". 5408 Changes made in -05 were incomplete. This should allow closing 5409 Ticket #42. 5411 * Removed spurious "for use in delivery notifications" from 3.6.2. 5412 Was just a pasting-type error. 5414 * Changed "In other words" to "In particular" in Section 2.3.5 per 5415 Ticket #10 and July 2021 mailing list discussion. Removed 5416 associated CREF. 5418 * Converted to xml2rfc v3 (thanks to John Levine for doing the hard 5419 parts) and then modified the introductory note accordingly. 5421 * Started reworking the Abstract -- see revised CREF there. 5423 * Rewrote Section 2.3.3 slightly to note the existence of submission 5424 servers and removed the CREF. 5426 * Updated Appendix G.7.17 and slightly modified CREF note in 5427 Section 2 -- proposed to not get 5321bis involved with this 5428 (Ticket #50). 5430 * Rewrote parts of Section 3.4.2 to clarify text amd respond to 5431 Ticket #34. 5433 * Inserted suggested text info CREF at end of Section 1.2. Comments 5434 welcome. Soon. 5436 H.3.11. Changes from draft-ietf-emailcore-rfc5321bis-06 (2021-11-07) to 5437 -07 5439 * Reviewed closed tickets and discussion with co-chairs after IETF 5440 112 and updated text. Sections or items that are, according to 5441 the ticket list, completely closed have been identified by 5442 "(closed)" in or near their titles. 5444 * Changed the suggestion for references to other documents mentioned 5445 in G.7.14 and Section 1.2 to actual text. Cleaned things up and, 5446 per note from Alexey 2021-11-17, have marked Ticket #53 as closed. 5448 * New text added and old text replaced about quotes in 5449 Section 4.1.2, text rearranged and edited a bit per Appendix G.9, 5450 and CREF added about alternatives. Changes reflect mailing list 5451 comments through 5453 * Last sentence (about source routing) removed from Section 2.1. 5454 Also adjusted text in Section 3.3, Section 4.1.1.3 but work is 5455 still needed there (see new CREFs in that section) and 5456 Section 6.1. The former Appendix C and references to it have been 5457 removed, leaving a placeholder to avoid changing subsequent 5458 appendix numbering before IETF Last Call (and maybe its 5459 completion) No changes have yet been made to Appendix F.2 but it 5460 is likely to require some work in the next version of the 5461 document. This is entirely about Ticket #17, which should not be 5462 closed until that appendix is updated. 5464 H.3.12. Changes from draft-ietf-emailcore-rfc5321bis-07 (2021-12-04) to 5465 -08 5467 Other than the partial cleanup for "forwarding" and "aliasing" and 5468 miscellaneous editorial fixes and corrections (including cleaning out 5469 unused references), changes in this version reflect the conclusions 5470 of the EMAILCORE interim meeting held 2021-12-09. References to 5471 "slides" are to the deck at https://datatracker.ietf.org/doc/slides- 5472 interim-2021-emailcore-01-sessa-chairs-slides/ and the minutes at 5473 https://notes.ietf.org/notes-emailcore-interim-dec-2021 5475 * (Slides 9 through 12): Removed source route examples from 5476 Section 4.1.1.3 and added a new paragraph explaining what happened 5477 to them. For slides 11 and 12, see below for more general 5478 Appendix F.2 discussion. 5479 (Cf Appendix G.7.10 and Ticket #17.) 5481 * (Slides 13 through 14): Domain names, Section 2.3.5. Removed 5482 "resolvable". Changed "alias" to "host alias" (although, after 5483 looking at the actual text, the intent seems clear from the CNAME 5484 label comment and, of course, the term "host" has been 5485 controversial in DNS circles and the minutes are not clear on the 5486 desirability of this change). Inserted "MUST" for the FQDN. A 5487 cross-reference to the domain name discussion in this section has 5488 been added to Section 4.1.1.1 in an attempt to resolve that 5489 discussion. 5490 In going carefully through this material, it became obvious that 5491 the discussions in Section 2.3.5 and Section 5 were confusing and 5492 somewhat redundant. Those sections have been rewritten to clarify 5493 intent, hint that extensions may modify (or have modified) a few 5494 of the rules, improve cross-references, and remove redundant text. 5495 Domain name issues are still under discussion on the WG mailing 5496 list as of 2021-12-18 and it is possible that the above changes 5497 may have introduced new issues, so additional changes are 5498 possible. 5499 (Cf target="G-domain"/> and Tickets #9 and maybe #10.) 5501 * Aliasing and forwarding: 5502 Consolidated former sections 3.4 and 3.9 into a new Section 3.4, 5503 making them subsections. The new subsection probably still needs 5504 work and maybe an introductory paragraph, but even bringing the 5505 two subsections together may reduce some sources of confusion 5506 identified on the mailing list. Added cross-reference to security 5507 considerations from the new Section 3.4.1. 5509 All other issues discussed during the interim appear to be unresolved 5510 and were deferred to the mailing list. 5512 As what should be the third and final step in deprecation of source 5513 routes and removal of them from the main text, the appendix that 5514 discusses them (Appendix F.2) has been rewritten, adjusting language 5515 and incorporating some materials from the former Appendix C. 5517 Index 5519 A C S 5521 A 5523 Argument Syntax 5524 ALPHA Section 4.1.2, Paragraph 2, Item 1 5525 Additional-Registered-Clauses Section 4.4.1 5526 Addtl-Link Section 4.4.1 5527 Addtl-Protocol Section 4.4.1 5528 Argument Section 4.1.2 5529 Atom Section 4.1.2 5530 By-domain Section 4.4.1 5531 CFWS Section 4.1.2, Paragraph 2, Item 2 5532 CRLF Section 4.1.2, Paragraph 2, Item 1 5533 DIGIT Section 4.1.2, Paragraph 2, Item 1 5534 Domain Section 4.1.2 5535 Dot-string Section 4.1.2 5536 Extended-Domain Section 4.4.1 5537 FWS Section 4.1.2, Paragraph 2, Item 2 5538 For Section 4.4.1 5539 Forward-Path Section 4.1.2 5540 From-domain Section 4.4.1 5541 General-address-literal Section 4.1.3 5542 Greeting Section 4.2 5543 HEXDIG Section 4.1.2, Paragraph 2, Item 1 5544 ID Section 4.4.1 5545 IPv4-address-literal Section 4.1.3 5546 IPv6-addr Section 4.1.3 5547 IPv6-address-literal Section 4.1.3 5548 Keyword Section 4.1.2 5549 Ldh-str Section 4.1.2 5550 Let-dig Section 4.1.2 5551 Link Section 4.4.1 5552 Local-part Section 4.1.2 5553 Mail-parameters Section 4.1.2 5554 Mailbox Section 4.1.2 5555 Opt-info Section 4.4.1 5556 Path Section 4.1.2 5557 Protocol Section 4.4.1 5558 QcontentSMTP Section 4.1.2 5559 Quoted-string Section 4.1.2 5560 Rcpt-parameters Section 4.1.2 5561 Reply-code Section 4.2 5562 Reply-line Section 4.2 5563 Return-path-line Section 4.4.1 5564 Reverse-Path Section 4.1.2 5565 SP Section 4.1.2, Paragraph 2, Item 1 5566 Snum Section 4.1.3 5567 Stamp Section 4.4.1 5568 Standardized-tag Section 4.1.3 5569 String Section 4.1.2 5570 TCP-info Section 4.4.1 5571 Time-stamp-line Section 4.4.1 5572 Via Section 4.4.1 5573 With Section 4.4.1 5574 address-literal Section 4.1.2 5575 atext Section 4.1.2, Paragraph 2, Item 2 5576 dcontent Section 4.1.3 5577 esmtp-keyword Section 4.1.2 5578 esmtp-param Section 4.1.2 5579 esmtp-value Section 4.1.2 5580 h16 Section 4.1.3 5581 ls32 Section 4.1.3 5582 qtextSMTP Section 4.1.2 5583 quoted-pairSMTP Section 4.1.2 5584 sub-domain Section 4.1.2 5585 textstring Section 4.2 5587 C 5589 Command Syntax 5590 data Section 4.1.1.4, Paragraph 8, Item 1 5591 ehlo Section 3.2, Paragraph 1; Section 4.1.1.1, Paragraph 1 5592 expn Section 4.1.1.7, Paragraph 4, Item 1 5593 helo Section 4.1.1.1, Paragraph 1 5594 help Section 4.1.1.8, Paragraph 5, Item 1 5595 mail Section 4.1.1.2 5596 noop Section 4.1.1.9, Paragraph 4, Item 1 5597 quit Section 4.1.1.10, Paragraph 5, Item 1 5598 rcpt Section 4.1.1.3, Paragraph 15 5599 rset Section 4.1.1.5, Paragraph 4, Item 1 5600 send, saml, soml Appendix G.7.13, Paragraph 1 5601 vrfy Section 4.1.1.6, Paragraph 4, Item 1 5603 S 5605 Source Routes Appendix F.2 5606 A-d-l Appendix F.2 5607 At-domain Appendix F.2 5608 Path Appendix F.2 5610 Author's Address 5612 John C. Klensin 5613 1770 Massachusetts Ave, Suite 322 5614 Cambridge, MA 02140 5615 United States of America 5617 Email: john-ietf@jck.com