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'12') (Obsoleted by RFC 2822) -- Obsolete informational reference (is this intentional?): RFC 974 (ref. '15') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 1869 (ref. '21') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 2821 (ref. '29') (Obsoleted by RFC 5321) -- Obsolete informational reference (is this intentional?): RFC 3501 (ref. '36') (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 3798 (ref. '37') (Obsoleted by RFC 8098) -- Obsolete informational reference (is this intentional?): RFC 3851 (ref. '38') (Obsoleted by RFC 5751) -- Obsolete informational reference (is this intentional?): RFC 4408 (ref. '41') (Obsoleted by RFC 7208) -- Obsolete informational reference (is this intentional?): RFC 4409 (ref. '42') (Obsoleted by RFC 6409) -- Obsolete informational reference (is this intentional?): RFC 4871 (ref. '44') (Obsoleted by RFC 6376) Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 18 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMAILCORE J. Klensin 3 Internet-Draft December 25, 2020 4 Obsoletes: 5321, 1846, 7504 (if 5 approved) 6 Updates: 1123 (if approved) 7 Intended status: Standards Track 8 Expires: June 28, 2021 10 Simple Mail Transfer Protocol 11 draft-ietf-emailcore-rfc5321bis-01 13 Abstract 15 This document is a specification of the basic protocol for Internet 16 electronic mail transport. It consolidates, updates, and clarifies 17 several previous documents, making all or parts of most of them 18 obsolete. It covers the SMTP extension mechanisms and best practices 19 for the contemporary Internet, but does not provide details about 20 particular extensions. Although SMTP was designed as a mail 21 transport and delivery protocol, this specification also contains 22 information that is important to its use as a "mail submission" 23 protocol for "split-UA" (User Agent) mail reading systems and mobile 24 environments. This document replaces the earlier version with the 25 same title in RFC 5321. 26 [[CREF1: Note in Draft: Except for the last sentence, the above is 27 unchanged from 5321 and may need adjusting in the light of RFC 6409 28 as an Internet Standard.]] 30 Note on Reading This Working Draft 32 This working draft is extensively annotated with information about 33 changes made over the decade since RFC 5321 appeared, especially when 34 those changes might be controversial or should get careful review. 35 Anything marked in CREF comments with "[5321bis]" is current. In 36 general, unless those are marked with "[[Note in Draft", in the 37 contents of an "Editor's note", or are in the "Errata Summary" 38 appendix (Appendix H.1, they are just notes on changes that have 39 already been made and where those changes originated. Comments 40 identified as "2821ter" arose after the Last Call on what became 41 RFC5321, sometimes before AUTH48 on that document or a bit later. 42 Those, of course, should still be reviewed. Surviving comments about 43 rfc5321bis-00 followed by a letter indicate intermediate working 44 versions of this draft and can be ignored unless the origin of 45 changes is important. As one can tell from the dates (when they are 46 given), this document has been periodically updated over a very long 47 period of time. 49 This evolving draft should be discussed on the ietf-smtp@ietf.org 50 list. 52 Status of This Memo 54 This Internet-Draft is submitted in full conformance with the 55 provisions of BCP 78 and BCP 79. 57 Internet-Drafts are working documents of the Internet Engineering 58 Task Force (IETF). Note that other groups may also distribute 59 working documents as Internet-Drafts. The list of current Internet- 60 Drafts is at https://datatracker.ietf.org/drafts/current/. 62 Internet-Drafts are draft documents valid for a maximum of six months 63 and may be updated, replaced, or obsoleted by other documents at any 64 time. It is inappropriate to use Internet-Drafts as reference 65 material or to cite them other than as "work in progress." 67 This Internet-Draft will expire on June 28, 2021. 69 Copyright Notice 71 Copyright (c) 2020 IETF Trust and the persons identified as the 72 document authors. All rights reserved. 74 This document is subject to BCP 78 and the IETF Trust's Legal 75 Provisions Relating to IETF Documents 76 (https://trustee.ietf.org/license-info) in effect on the date of 77 publication of this document. Please review these documents 78 carefully, as they describe your rights and restrictions with respect 79 to this document. Code Components extracted from this document must 80 include Simplified BSD License text as described in Section 4.e of 81 the Trust Legal Provisions and are provided without warranty as 82 described in the Simplified BSD License. 84 Table of Contents 86 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 87 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . 6 88 1.2. History and Context for This Document . . . . . . . . . . 6 89 1.3. Document Conventions . . . . . . . . . . . . . . . . . . 7 90 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . 8 91 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 8 92 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 10 93 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . 10 94 2.2.2. Definition and Registration of Extensions . . . . . . 11 95 2.2.3. Special Issues with Extensions . . . . . . . . . . . 12 96 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . 13 97 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . 13 98 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 13 99 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . 13 100 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . 14 101 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . 14 102 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . 15 103 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . 15 104 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 16 105 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 16 106 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . 16 107 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 17 108 2.4. General Syntax Principles and Transaction Model . . . . . 17 109 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . 19 110 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 19 111 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 20 112 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 20 113 3.4. Forwarding for Address Correction or Updating . . . . . . 23 114 3.5. Commands for Debugging Addresses . . . . . . . . . . . . 24 115 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 24 116 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . 26 117 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . 27 118 3.5.4. Semantics and Applications of EXPN . . . . . . . . . 27 119 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 27 120 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . 27 121 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . 28 122 3.6.3. Message Submission Servers as Relays . . . . . . . . 29 123 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 30 124 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 30 125 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 30 126 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 31 127 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 31 128 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 31 129 3.8. Terminating Sessions and Connections . . . . . . . . . . 31 130 3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 32 131 3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 33 132 3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . 33 133 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 33 134 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 33 135 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . 33 136 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 42 137 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . 44 138 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 46 139 4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . 47 140 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 48 141 4.2.1. Reply Code Severities and Theory . . . . . . . . . . 49 142 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . 52 143 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . 53 144 4.2.4. Some specific code situations and relationships . . . 55 146 4.3. Sequencing of Commands and Replies . . . . . . . . . . . 56 147 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 56 148 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 57 149 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 59 150 4.5. Additional Implementation Issues . . . . . . . . . . . . 63 151 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . 63 152 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . 64 153 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . 65 154 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . 69 155 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 71 156 5. Address Resolution and Mail Handling . . . . . . . . . . . . 71 157 5.1. Locating the Target Host . . . . . . . . . . . . . . . . 72 158 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 74 159 6. Problem Detection and Handling . . . . . . . . . . . . . . . 74 160 6.1. Reliable Delivery and Replies by Email . . . . . . . . . 74 161 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . 75 162 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . 76 163 6.4. Compensating for Irregularities . . . . . . . . . . . . . 76 164 7. Security Considerations . . . . . . . . . . . . . . . . . . . 78 165 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 78 166 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 79 167 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . 79 168 7.4. Mail Rerouting Based on the 251 and 551 Response 169 Codes . . . . . . . . . . . . . . . . . . . . . . . . . . 80 170 7.5. Information Disclosure in Announcements . . . . . . . . . 80 171 7.6. Information Disclosure in Trace Fields . . . . . . . . . 81 172 7.7. Information Disclosure in Message Forwarding . . . . . . 81 173 7.8. Resistance to Attacks . . . . . . . . . . . . . . . . . . 81 174 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 81 175 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 82 176 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 83 177 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 84 178 10.1. Normative References . . . . . . . . . . . . . . . . . . 84 179 10.2. Informative References . . . . . . . . . . . . . . . . . 85 180 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 90 181 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 90 182 Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . 91 183 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 92 184 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 92 185 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 93 186 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 94 187 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 95 188 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 96 189 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 96 190 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 96 191 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 96 192 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 97 193 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 97 194 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 97 195 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 97 196 Appendix G. Other Outstanding Issues . . . . . . . . . . . . . . 98 197 G.1. IP Address literals . . . . . . . . . . . . . . . . . . . 99 198 G.2. Repeated Use of EHLO . . . . . . . . . . . . . . . . . . 99 199 G.3. Meaning of "MTA" and Related Terminology . . . . . . . . 99 200 G.4. Originator, or Originating System, Authentication . . . . 100 201 G.5. Remove or deprecate the work-around from code 552 to 452 100 202 G.6. Clarify where the protocol stands with respect to 203 submission and TLS issues . . . . . . . . . . . . . . . . 100 204 G.7. Probably-substantive Discussion Topics Identified in 205 Other Ways . . . . . . . . . . . . . . . . . . . . . . . 100 206 G.7.1. Issues with 521, 554, and 556 codes . . . . . . . . . 100 207 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 101 208 G.7.3. Resolvable FQDNs and private domain names . . . . . . 101 209 G.7.4. Possible clarification about mail transactions and 210 transaction state . . . . . . . . . . . . . . . . . . 101 211 G.7.5. Issues with mailing lists, aliases, and forwarding . 101 212 G.7.6. Requirements for domain name and/or IP address in 213 EHLO . . . . . . . . . . . . . . . . . . . . . . . . 101 214 G.7.7. Does the 'first digit only' and/or non-listed reply 215 code text need clarification? . . . . . . . . . . . . 101 216 G.7.8. Size limits . . . . . . . . . . . . . . . . . . . . . 101 217 G.7.9. Discussion of 'blind' copies and RCPT . . . . . . . . 102 218 G.7.10. Further clarifications needed to source routes? . . . 102 219 G.7.11. Should 1yz Be Revisited? . . . . . . . . . . . . . . 102 220 G.7.12. Review Timeout Specifications . . . . . . . . . . . . 102 221 G.7.13. Possible SEND, SAML, SOML Loose End . . . . . . . . . 102 222 G.8. Enhanced Reply Codes and DSNs . . . . . . . . . . . . . . 102 223 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 103 224 G.10. Internationalization . . . . . . . . . . . . . . . . . . 103 225 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 104 226 G.12. Extension Keywords Starting in 'X-' . . . . . . . . . . . 104 227 G.13. Deprecating HELO . . . . . . . . . . . . . . . . . . . . 104 228 Appendix H. RFC 5321 Errata Summary and Tentative Change Log . . 104 229 H.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 104 230 H.2. Changes from RFC 5321 (published October 2008) to the 231 initial (-00) version of this draft . . . . . . . . . . . 106 232 H.3. Changes Among Versions of Rfc5321bis . . . . . . . . . . 107 233 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 234 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 107 235 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) 236 to -02 . . . . . . . . . . . . . . . . . . . . . . . 107 237 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 238 to -03 . . . . . . . . . . . . . . . . . . . . . . . 108 239 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) 240 to draft-ietf-emailcore-rfc5321bis-00 . . . . . . . . 108 241 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 242 (2020-10-06) to -01 . . . . . . . . . . . . . . . . . 108 243 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 244 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 111 246 1. Introduction 248 1.1. Transport of Electronic Mail 250 The objective of the Simple Mail Transfer Protocol (SMTP) is to 251 transfer mail reliably and efficiently. 253 SMTP is independent of the particular transmission subsystem and 254 requires only a reliable ordered data stream channel. While this 255 document specifically discusses transport over TCP, other transports 256 are possible. Appendices to RFC 821 [3] describe some of them. 258 An important feature of SMTP is its capability to transport mail 259 across multiple networks, usually referred to as "SMTP mail relaying" 260 (see Section 3.6). A network consists of the mutually-TCP-accessible 261 hosts on the public Internet, the mutually-TCP-accessible hosts on a 262 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 263 environment utilizing a non-TCP transport-level protocol. Using 264 SMTP, a process can transfer mail to another process on the same 265 network or to some other network via a relay or gateway process 266 accessible to both networks. 268 In this way, a mail message may pass through a number of intermediate 269 relay or gateway hosts on its path from sender to ultimate recipient. 270 The Mail eXchanger mechanisms of the domain name system (RFC 1035 271 [4], RFC 974 [15], and Section 5 of this document) are used to 272 identify the appropriate next-hop destination for a message being 273 transported. 275 1.2. History and Context for This Document 277 This document is a specification of the basic protocol for the 278 Internet electronic mail transport. It consolidates, updates and 279 clarifies, but does not add new or change existing functionality of 280 the following: 282 o the original SMTP (Simple Mail Transfer Protocol) specification of 283 RFC 821 [3], 285 o domain name system requirements and implications for mail 286 transport from RFC 1035 [4] and RFC 974 [15], 288 o the clarifications and applicability statements in RFC 1123 [5], 289 o the new error codes added by RFC 1846 [19] and later by RFC 7504 290 [48], obsoleting both of those documents, and 292 o material drawn from the SMTP Extension mechanisms in RFC 1869 293 [21]. 295 o Editorial and clarification changes to RFC 2821 [29] to bring that 296 specification to Draft Standard. 298 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC 299 1123 (replacing the mail transport materials of RFC 1123). However, 300 RFC 821 specifies some features that were not in significant use in 301 the Internet by the mid-1990s and (in appendices) some additional 302 transport models. Those sections are omitted here in the interest of 303 clarity and brevity; readers needing them should refer to RFC 821. 305 It also includes some additional material from RFC 1123 that required 306 amplification. This material has been identified in multiple ways, 307 mostly by tracking flaming on various lists and newsgroups and 308 problems of unusual readings or interpretations that have appeared as 309 the SMTP extensions have been deployed. Where this specification 310 moves beyond consolidation and actually differs from earlier 311 documents, it supersedes them technically as well as textually. 313 Although SMTP was designed as a mail transport and delivery protocol, 314 this specification also contains information that is important to its 315 use as a "mail submission" protocol, as recommended for Post Office 316 Protocol (POP) (RFC 937 [13], RFC 1939 [22]) and IMAP (RFC 3501 317 [36]). In general, the separate mail submission protocol specified 318 in RFC 4409 [42] is now preferred to direct use of SMTP; more 319 discussion of that subject appears in that document. 321 Section 2.3 provides definitions of terms specific to this document. 322 Except when the historical terminology is necessary for clarity, this 323 document uses the current 'client' and 'server' terminology to 324 identify the sending and receiving SMTP processes, respectively. 326 A companion document, RFC 5322 [11], discusses message header 327 sections and bodies and specifies formats and structures for them. 329 1.3. Document Conventions 331 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 332 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 333 document are to be interpreted as described in RFC 2119 [1]. As each 334 of these terms was intentionally and carefully chosen to improve the 335 interoperability of email, each use of these terms is to be treated 336 as a conformance requirement. 338 Because this document has a long history and to avoid the risk of 339 various errors and of confusing readers and documents that point to 340 this one, most examples and the domain names they contain are 341 preserved from RFC 2821. Readers are cautioned that these are 342 illustrative examples that should not actually be used in either code 343 or configuration files. 345 2. The SMTP Model 347 [[CREF2: [5321bis] [[Editor's Note: There have been extensive and 348 repeated discussions on the SMTP and IETF lists about whether this 349 document should say something about hop-by-hop (MTA-to-MTA) SMTP 350 authentication and, if so, what?? Note that end to end message 351 authentication is almost certainly out of scope for SMTP.]]]] 353 2.1. Basic Structure 355 The SMTP design can be pictured as: 357 +----------+ +----------+ 358 +------+ | | | | 359 | User |<-->| | SMTP | | 360 +------+ | Client- |Commands/Replies| Server- | 361 +------+ | SMTP |<-------------->| SMTP | +------+ 362 | File |<-->| | and Mail | |<-->| File | 363 |System| | | | | |System| 364 +------+ +----------+ +----------+ +------+ 365 SMTP client SMTP server 367 When an SMTP client has a message to transmit, it establishes a two- 368 way transmission channel to an SMTP server. The responsibility of an 369 SMTP client is to transfer mail messages to one or more SMTP servers, 370 or report its failure to do so. 372 The means by which a mail message is presented to an SMTP client, and 373 how that client determines the identifier(s) ("names") of the 374 domain(s) to which mail messages are to be transferred, are local 375 matters. They are not addressed by this document. In some cases, 376 the designated domain(s), or those determined by an SMTP client, will 377 identify the final destination(s) of the mail message. In other 378 cases, common with SMTP clients associated with implementations of 379 the POP (RFC 937 [13], RFC 1939 [22]) or IMAP (RFC 3501 [36]) 380 protocols, or when the SMTP client is inside an isolated transport 381 service environment, the domain determined will identify an 382 intermediate destination through which all mail messages are to be 383 relayed. SMTP clients that transfer all traffic regardless of the 384 target domains associated with the individual messages, or that do 385 not maintain queues for retrying message transmissions that initially 386 cannot be completed, may otherwise conform to this specification but 387 are not considered fully-capable. Fully-capable SMTP 388 implementations, including the relays used by these less capable 389 ones, and their destinations, are expected to support all of the 390 queuing, retrying, and alternate address functions discussed in this 391 specification. In many situations and configurations, the less- 392 capable clients discussed above SHOULD be using the message 393 submission protocol (RFC 4409 [42]) rather than SMTP. 395 The means by which an SMTP client, once it has determined a target 396 domain, determines the identity of an SMTP server to which a copy of 397 a message is to be transferred, and then performs that transfer, are 398 covered by this document. To effect a mail transfer to an SMTP 399 server, an SMTP client establishes a two-way transmission channel to 400 that SMTP server. An SMTP client determines the address of an 401 appropriate host running an SMTP server by resolving a destination 402 domain name to either an intermediate Mail eXchanger host or a final 403 target host. 405 An SMTP server may be either the ultimate destination or an 406 intermediate "relay" (that is, it may assume the role of an SMTP 407 client after receiving the message) or "gateway" (that is, it may 408 transport the message further using some protocol other than SMTP). 409 SMTP commands are generated by the SMTP client and sent to the SMTP 410 server. SMTP replies are sent from the SMTP server to the SMTP 411 client in response to the commands. 413 In other words, message transfer can occur in a single connection 414 between the original SMTP-sender and the final SMTP-recipient, or can 415 occur in a series of hops through intermediary systems. In either 416 case, once the server has issued a success response at the end of the 417 mail data, a formal handoff of responsibility for the message occurs: 418 the protocol requires that a server MUST accept responsibility for 419 either delivering the message or properly reporting the failure to do 420 so (see Sections 6.1, 6.2, and 7.8, below). 422 Once the transmission channel is established and initial handshaking 423 is completed, the SMTP client normally initiates a mail transaction. 424 Such a transaction consists of a series of commands to specify the 425 originator and destination of the mail and transmission of the 426 message content (including any lines in the header section or other 427 structure) itself. When the same message is sent to multiple 428 recipients, this protocol encourages the transmission of only one 429 copy of the data for all recipients at the same destination (or 430 intermediate relay) host. 432 The server responds to each command with a reply; replies may 433 indicate that the command was accepted, that additional commands are 434 expected, or that a temporary or permanent error condition exists. 435 Commands specifying the sender or recipients may include server- 436 permitted SMTP service extension requests, as discussed in 437 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 438 although this can be modified by mutually agreed upon extension 439 requests such as command pipelining (RFC 2920 [30]). 441 Once a given mail message has been transmitted, the client may either 442 request that the connection be shut down or may initiate other mail 443 transactions. In addition, an SMTP client may use a connection to an 444 SMTP server for ancillary services such as verification of email 445 addresses or retrieval of mailing list subscriber addresses. 447 As suggested above, this protocol provides mechanisms for the 448 transmission of mail. Historically, this transmission normally 449 occurred directly from the sending user's host to the receiving 450 user's host when the two hosts are connected to the same transport 451 service. When they are not connected to the same transport service, 452 transmission occurs via one or more relay SMTP servers. A very 453 common case in the Internet today involves submission of the original 454 message to an intermediate, "message submission" server, which is 455 similar to a relay but has some additional properties; such servers 456 are discussed in Section 2.3.10 and at some length in RFC 4409 [42]. 457 An intermediate host that acts as either an SMTP relay or as a 458 gateway into some other transmission environment is usually selected 459 through the use of the domain name service (DNS) Mail eXchanger 460 mechanism. Explicit "source" routing (see Section 5 and Appendix C 461 and Appendix F.2) SHOULD NOT be used. [[CREF3: [5321bis] JcK 462 20090123 - redundant sentence removed.]] 464 2.2. The Extension Model 466 2.2.1. Background 468 In an effort that started in 1990, approximately a decade after RFC 469 821 was completed, the protocol was modified with a "service 470 extensions" model that permits the client and server to agree to 471 utilize shared functionality beyond the original SMTP requirements. 472 The SMTP extension mechanism defines a means whereby an extended SMTP 473 client and server may recognize each other, and the server can inform 474 the client as to the service extensions that it supports. 476 Contemporary SMTP implementations MUST support the basic extension 477 mechanisms. For instance, servers MUST support the EHLO command even 478 if they do not implement any specific extensions and clients SHOULD 479 preferentially utilize EHLO rather than HELO. (However, for 480 compatibility with older conforming implementations, SMTP clients and 481 servers MUST support the original HELO mechanisms as a fallback.) 482 Unless the different characteristics of HELO must be identified for 483 interoperability purposes, this document discusses only EHLO. 485 SMTP is widely deployed and high-quality implementations have proven 486 to be very robust. However, the Internet community now considers 487 some services to be important that were not anticipated when the 488 protocol was first designed. If support for those services is to be 489 added, it must be done in a way that permits older implementations to 490 continue working acceptably. The extension framework consists of: 492 o The SMTP command EHLO, superseding the earlier HELO, 494 o a registry of SMTP service extensions, 496 o additional parameters to the SMTP MAIL and RCPT commands, and 498 o optional replacements for commands defined in this protocol, such 499 as for DATA in non-ASCII transmissions (RFC 3030 [32]). 501 SMTP's strength comes primarily from its simplicity. Experience with 502 many protocols has shown that protocols with few options tend towards 503 ubiquity, whereas protocols with many options tend towards obscurity. 505 Each and every extension, regardless of its benefits, must be 506 carefully scrutinized with respect to its implementation, deployment, 507 and interoperability costs. In many cases, the cost of extending the 508 SMTP service will likely outweigh the benefit. 510 2.2.2. Definition and Registration of Extensions 512 The IANA maintains a registry of SMTP service extensions. A 513 corresponding EHLO keyword value is associated with each extension. 514 Each service extension registered with the IANA must be defined in a 515 formal Standards-Track or IESG-approved Experimental protocol 516 document. The definition must include: 518 o the textual name of the SMTP service extension; 520 o the EHLO keyword value associated with the extension; 522 o the syntax and possible values of parameters associated with the 523 EHLO keyword value; 525 o any additional SMTP verbs associated with the extension 526 (additional verbs will usually be, but are not required to be, the 527 same as the EHLO keyword value); 529 o any new parameters the extension associates with the MAIL or RCPT 530 verbs; 532 o a description of how support for the extension affects the 533 behavior of a server and client SMTP; and 535 o the increment by which the extension is increasing the maximum 536 length of the commands MAIL and/or RCPT, over that specified in 537 this Standard. 539 In addition, any EHLO keyword value starting with an upper or lower 540 case "X" refers to a local SMTP service extension used exclusively 541 through bilateral agreement. Keywords beginning with "X" MUST NOT be 542 used in a registered service extension. Conversely, keyword values 543 presented in the EHLO response that do not begin with "X" MUST 544 correspond to a Standard, Standards-Track, or IESG-approved 545 Experimental SMTP service extension registered with IANA. A 546 conforming server MUST NOT offer non-"X"-prefixed keyword values that 547 are not described in a registered extension. 549 Additional verbs and parameter names are bound by the same rules as 550 EHLO keywords; specifically, verbs beginning with "X" are local 551 extensions that may not be registered or standardized. Conversely, 552 verbs not beginning with "X" must always be registered. 554 2.2.3. Special Issues with Extensions 556 Extensions that change fairly basic properties of SMTP operation are 557 permitted. The text in other sections of this document must be 558 understood in that context. In particular, extensions can change the 559 minimum limits specified in Section 4.5.3, can change the ASCII 560 character set requirement as mentioned above, or can introduce some 561 optional modes of message handling. 563 In particular, if an extension implies that the delivery path 564 normally supports special features of that extension, and an 565 intermediate SMTP system finds a next hop that does not support the 566 required extension, it MAY choose, based on the specific extension 567 and circumstances, to requeue the message and try later and/or try an 568 alternate MX host. If this strategy is employed, the timeout to fall 569 back to an unextended format (if one is available) SHOULD be less 570 than the normal timeout for bouncing as undeliverable (e.g., if 571 normal timeout is three days, the requeue timeout before attempting 572 to transmit the mail without the extension might be one day). 574 2.3. SMTP Terminology 576 2.3.1. Mail Objects 578 SMTP transports a mail object. A mail object contains an envelope 579 and content. 581 The SMTP envelope is sent as a series of SMTP protocol units 582 (described in Section 3). It consists of an originator address (to 583 which error reports should be directed), one or more recipient 584 addresses, and optional protocol extension material. Historically, 585 variations on the reverse-path (originator) address specification 586 command (MAIL) could be used to specify alternate delivery modes, 587 such as immediate display; those variations have now been deprecated 588 (see Appendix F and Appendix F.6). 590 The SMTP content is sent in the SMTP DATA protocol unit and has two 591 parts: the header section and the body. If the content conforms to 592 other contemporary standards, the header section consists of a 593 collection of header fields, each consisting of a header name, a 594 colon, and data, structured as in the message format specification 595 (RFC 5322 [11]); the body, if structured, is defined according to 596 MIME (RFC 2045 [24]). The content is textual in nature, expressed 597 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 598 "8BITMIME", RFC 6152 [47]) may relax this restriction for the content 599 body, the content header fields are always encoded using the US-ASCII 600 repertoire. Two MIME extensions (RFC 2047 [25] and RFC 2231 [28]) 601 define an algorithm for representing header values outside the US- 602 ASCII repertoire, while still encoding them using the US-ASCII 603 repertoire. 605 2.3.2. Senders and Receivers 607 In RFC 821, the two hosts participating in an SMTP transaction were 608 described as the "SMTP-sender" and "SMTP-receiver". This document 609 has been changed to reflect current industry terminology and hence 610 refers to them as the "SMTP client" (or sometimes just "the client") 611 and "SMTP server" (or just "the server"), respectively. Since a 612 given host may act both as server and client in a relay situation, 613 "receiver" and "sender" terminology is still used where needed for 614 clarity. 616 2.3.3. Mail Agents and Message Stores 618 Additional mail system terminology became common after RFC 821 was 619 published and, where convenient, is used in this specification. In 620 particular, SMTP servers and clients provide a mail transport service 621 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 622 Agents" (MUAs or UAs) are normally thought of as the sources and 623 targets of mail. At the source, an MUA might collect mail to be 624 transmitted from a user and hand it off to an MTA; the final 625 ("delivery") MTA would be thought of as handing the mail off to an 626 MUA (or at least transferring responsibility to it, e.g., by 627 depositing the message in a "message store"). However, while these 628 terms are used with at least the appearance of great precision in 629 other environments, the implied boundaries between MUAs and MTAs 630 often do not accurately match common, and conforming, practices with 631 Internet mail. Hence, the reader should be cautious about inferring 632 the strong relationships and responsibilities that might be implied 633 if these terms were used elsewhere. 635 2.3.4. Host 637 For the purposes of this specification, a host is a computer system 638 attached to the Internet (or, in some cases, to a private TCP/IP 639 network) and supporting the SMTP protocol. Hosts are known by names 640 (see the next section); they SHOULD NOT be identified by numerical 641 addresses, i.e., by address literals as described in Section 4.1.2. 643 2.3.5. Domain Names 645 A domain name (or often just a "domain") consists of one or more 646 components, separated by dots if more than one appears. In the case 647 of a top-level domain used by itself in an email address, a single 648 string is used without any dots. This makes the requirement, 649 described in more detail below, that only fully-qualified domain 650 names appear in SMTP transactions on the public Internet, 651 particularly important where top-level domains are involved. These 652 components ("labels" in DNS terminology, RFC 1035 [4]) are restricted 653 for SMTP purposes to consist of a sequence of letters, digits, and 654 hyphens drawn from the ASCII character set [2] and conforming to what 655 RFC 1035 Section 2.3.1 calls the "preferred name syntax". Domain 656 names are used as names of hosts and of other entities in the domain 657 name hierarchy. For example, a domain may refer to an alias (label 658 of a CNAME RR) or the label of Mail eXchanger records to be used to 659 deliver mail instead of representing a host name. See RFC 1035 [4] 660 and Section 5 of this specification. 662 The domain name, as described in this document and in RFC 1035 [4], 663 is the entire, fully-qualified name (often referred to as an "FQDN"). 664 A domain name that is not in FQDN form is no more than a local alias. 665 Local aliases MUST NOT appear in any SMTP transaction. 667 Only resolvable, fully-qualified domain names (FQDNs) are permitted 668 when domain names are used in SMTP. 670 [[CREF4: [[5321bis Editor's Note: does "in the public DNS" or 671 equivalent need to be added to "resolvable"???]]]] 672 In other words, names that can be resolved to MX RRs or address 673 (i.e., A or AAAA) RRs (as discussed in Section 5) are permitted, as 674 are CNAME RRs whose targets can be resolved, in turn, to MX or 675 address RRs. 676 [[CREF5: [[5321bis Editor's Note: it is not clear whether "In other 677 words" really meant "for example" or it is was intended that the only 678 labels permitted are those that own records in one of the above RR 679 types]]]] 680 [[CREF6: [[5321bis Editor's Note: More generally, does this section 681 need work to clarify the relationship to private domain names 682 (discussed on SMTP list starting 2013-03-26)]]]] 683 Local nicknames or unqualified names MUST NOT be used. There are two 684 exceptions to the rule requiring FQDNs: 686 o The domain name given in the EHLO command MUST be either a primary 687 host name (a domain name that resolves to an address RR) or, if 688 the host has no name, an address literal, as described in 689 Section 4.1.3 and discussed further in the EHLO discussion of 690 Section 4.1.4. 692 o The reserved mailbox name "postmaster" may be used in a RCPT 693 command without domain qualification (see Section 4.1.1.3) and 694 MUST be accepted if so used. 696 2.3.6. Buffer and State Table 698 SMTP sessions are stateful, with both parties carefully maintaining a 699 common view of the current state. In this document, we model this 700 state by a virtual "buffer" and a "state table" on the server that 701 may be used by the client to, for example, "clear the buffer" or 702 "reset the state table", causing the information in the buffer to be 703 discarded and the state to be returned to some previous state. 705 2.3.7. Commands and Replies 707 SMTP commands and, unless altered by a service extension, message 708 data, are transmitted from the sender to the receiver via the 709 transmission channel in "lines". 711 An SMTP reply is an acknowledgment (positive or negative) sent in 712 "lines" from receiver to sender via the transmission channel in 713 response to a command. The general form of a reply is a numeric 714 completion code (indicating failure or success) usually followed by a 715 text string. The codes are for use by programs and the text is 716 usually intended for human users. RFC 3463 [34], specifies further 717 structuring of the reply strings, including the use of supplemental 718 and more specific completion codes (see also RFC 5248 [46]). 720 2.3.8. Lines 722 Lines consist of zero or more data characters terminated by the 723 sequence ASCII character "CR" (hex value 0D) followed immediately by 724 ASCII character "LF" (hex value 0A). This termination sequence is 725 denoted as in this document. Conforming implementations MUST 726 NOT recognize or generate any other character or character sequence 727 as a line terminator. Limits MAY be imposed on line lengths by 728 servers (see Section 4). 730 In addition, the appearance of "bare" "CR" or "LF" characters in text 731 (i.e., either without the other) has a long history of causing 732 problems in mail implementations and applications that use the mail 733 system as a tool. SMTP client implementations MUST NOT transmit 734 these characters except when they are intended as line terminators 735 and then MUST, as indicated above, transmit them only as a 736 sequence. 738 2.3.9. Message Content and Mail Data 740 The terms "message content" and "mail data" are used interchangeably 741 in this document to describe the material transmitted after the DATA 742 command is accepted and before the end of data indication is 743 transmitted. Message content includes the message header section and 744 the possibly structured message body. The MIME specification (RFC 745 2045 [24]) provides the standard mechanisms for structured message 746 bodies. 748 2.3.10. Originator, Delivery, Relay, and Gateway Systems 750 This specification makes a distinction among four types of SMTP 751 systems, based on the role those systems play in transmitting 752 electronic mail. An "originating" system (sometimes called an SMTP 753 originator) introduces mail into the Internet or, more generally, 754 into a transport service environment. A "delivery" SMTP system is 755 one that receives mail from a transport service environment and 756 passes it to a mail user agent or deposits it in a message store that 757 a mail user agent is expected to subsequently access. A "relay" SMTP 758 system (usually referred to just as a "relay") receives mail from an 759 SMTP client and transmits it, without modification to the message 760 data other than adding trace information, to another SMTP server for 761 further relaying or for delivery. 763 A "gateway" SMTP system (usually referred to just as a "gateway") 764 receives mail from a client system in one transport environment and 765 transmits it to a server system in another transport environment. 766 Differences in protocols or message semantics between the transport 767 environments on either side of a gateway may require that the gateway 768 system perform transformations to the message that are not permitted 769 to SMTP relay systems. For the purposes of this specification, 770 firewalls that rewrite addresses should be considered as gateways, 771 even if SMTP is used on both sides of them (see RFC 2979 [31]). 772 [[CREF7: [5321bis] [[Note in draft/Placeholder: There has been a 773 request to expand this section, possibly into a more extensive model 774 of Internet mail. Comments from others solicited. In particular, 775 does RFC 5598 make that suggestion OBE?]] ]] 777 2.3.11. Mailbox and Address 779 As used in this specification, an "address" is a character string 780 that identifies a user to whom mail will be sent or a location into 781 which mail will be deposited. The term "mailbox" refers to that 782 depository. The two terms are typically used interchangeably unless 783 the distinction between the location in which mail is placed (the 784 mailbox) and a reference to it (the address) is important. An 785 address normally consists of user and domain specifications. The 786 standard mailbox naming convention is defined to be "local- 787 part@domain"; contemporary usage permits a much broader set of 788 applications than simple "user names". Consequently, and due to a 789 long history of problems when intermediate hosts have attempted to 790 optimize transport by modifying them, the local-part MUST be 791 interpreted and assigned semantics only by the host specified in the 792 domain part of the address. 794 2.4. General Syntax Principles and Transaction Model 796 SMTP commands and replies have a rigid syntax. All commands begin 797 with a command verb. All replies begin with a three digit numeric 798 code. In some commands and replies, arguments are required following 799 the verb or reply code. Some commands do not accept arguments (after 800 the verb), and some reply codes are followed, sometimes optionally, 801 by free form text. In both cases, where text appears, it is 802 separated from the verb or reply code by a space character. Complete 803 definitions of commands and replies appear in Section 4. 805 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 806 and extension name keywords) are not case sensitive, with the sole 807 exception in this specification of a mailbox local-part (SMTP 808 Extensions may explicitly specify case-sensitive elements). That is, 809 a command verb, an argument value other than a mailbox local-part, 810 and free form text MAY be encoded in upper case, lower case, or any 811 mixture of upper and lower case with no impact on its meaning. The 812 local-part of a mailbox MUST BE treated as case sensitive. 814 Therefore, SMTP implementations MUST take care to preserve the case 815 of mailbox local-parts. In particular, for some hosts, the user 816 "smith" is different from the user "Smith". However, exploiting the 817 case sensitivity of mailbox local-parts impedes interoperability and 818 is discouraged. Mailbox domains follow normal DNS rules and are 819 hence not case sensitive. 821 A few SMTP servers, in violation of this specification (and RFC 821) 822 require that command verbs be encoded by clients in upper case. 823 Implementations MAY wish to employ this encoding to accommodate those 824 servers. 826 The argument clause consists of a variable-length character string 827 ending with the end of the line, i.e., with the character sequence 828 . The receiver will take no action until this sequence is 829 received. 831 The syntax for each command is shown with the discussion of that 832 command. Common elements and parameters are shown in Section 4.1.2. 834 Commands and replies are composed of characters from the ASCII 835 character set [2]. When the transport service provides an 8-bit byte 836 (octet) transmission channel, each 7-bit character is transmitted, 837 right justified, in an octet with the high-order bit cleared to zero. 838 More specifically, the unextended SMTP service provides 7-bit 839 transport only. An originating SMTP client that has not successfully 840 negotiated an appropriate extension with a particular server (see the 841 next paragraph) MUST NOT transmit messages with information in the 842 high-order bit of octets. If such messages are transmitted in 843 violation of this rule, receiving SMTP servers MAY clear the high- 844 order bit or reject the message as invalid. In general, a relay SMTP 845 SHOULD assume that the message content it has received is valid and, 846 assuming that the envelope permits doing so, relay it without 847 inspecting that content. Of course, if the content is mislabeled and 848 the data path cannot accept the actual content, this may result in 849 the ultimate delivery of a severely garbled message to the recipient. 850 Delivery SMTP systems MAY reject such messages, or return them as 851 undeliverable, rather than deliver them. In the absence of a server- 852 offered extension explicitly permitting it, a sending SMTP system is 853 not permitted to send envelope commands in any character set other 854 than US-ASCII. Receiving systems SHOULD reject such commands, 855 normally using "500 syntax error - invalid character" replies. 857 8-bit message content transmission MAY be requested of the server by 858 a client using extended SMTP facilities, notably the "8BITMIME" 859 extension, RFC 6152 [47]. 8BITMIME SHOULD be supported by SMTP 860 servers. However, it MUST NOT be construed as authorization to 861 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 862 transmission of any envelope material in other than ASCII. 8BITMIME 863 MUST NOT be requested by senders for material with the high bit on 864 that is not in MIME format with an appropriate content-transfer 865 encoding; servers MAY reject such messages. 867 The metalinguistic notation used in this document corresponds to the 868 "Augmented BNF" used in other Internet mail system documents. The 869 reader who is not familiar with that syntax should consult the ABNF 870 specification in RFC 5234 [10]. Metalanguage terms used in running 871 text are surrounded by pointed brackets (e.g., ) for clarity. 872 The reader is cautioned that the grammar expressed in the 873 metalanguage is not comprehensive. There are many instances in which 874 provisions in the text constrain or otherwise modify the syntax or 875 semantics implied by the grammar. 877 3. The SMTP Procedures: An Overview 879 This section contains descriptions of the procedures used in SMTP: 880 session initiation, mail transaction, forwarding mail, verifying 881 mailbox names and expanding mailing lists, and opening and closing 882 exchanges. Comments on relaying, a note on mail domains, and a 883 discussion of changing roles are included at the end of this section. 884 Several complete scenarios are presented in Appendix D. 886 3.1. Session Initiation 888 An SMTP session is initiated when a client opens a connection to a 889 server and the server responds with an opening message. 891 SMTP server implementations MAY include identification of their 892 software and version information in the connection greeting reply 893 after the 220 code, a practice that permits more efficient isolation 894 and repair of any problems. Implementations MAY make provision for 895 SMTP servers to disable the software and version announcement where 896 it causes security concerns. While some systems also identify their 897 contact point for mail problems, this is not a substitute for 898 maintaining the required "postmaster" address (see Section 4). 900 The SMTP protocol allows a server to formally reject a mail session 901 while still allowing the initial connection as follows: a 554 902 response MAY be given in the initial connection opening message 903 instead of the 220. A server taking this approach MUST still wait 904 for the client to send a QUIT (see Section 4.1.1.10) before closing 905 the connection and SHOULD respond to any intervening commands with 906 "503 bad sequence of commands". Since an attempt to make an SMTP 907 connection to such a system is probably in error, a server returning 908 a 554 response on connection opening SHOULD provide enough 909 information in the reply text to facilitate debugging of the sending 910 system. 912 3.2. Client Initiation 914 Once the server has sent the greeting (welcoming) message and the 915 client has received it, the client normally sends the EHLO command to 916 the server, indicating the client's identity. In addition to opening 917 the session, use of EHLO indicates that the client is able to process 918 service extensions and requests that the server provide a list of the 919 extensions it supports. Older SMTP systems that are unable to 920 support service extensions, and contemporary clients that do not 921 require service extensions in the mail session being initiated, MAY 922 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 923 style response to a HELO command. For a particular connection 924 attempt, if the server returns a "command not recognized" response to 925 EHLO, the client SHOULD be able to fall back and send HELO. 927 In the EHLO command, the host sending the command identifies itself; 928 the command may be interpreted as saying "Hello, I am " (and, 929 in the case of EHLO, "and I support service extension requests"). 931 3.3. Mail Transactions 933 There are three steps to SMTP mail transactions. The transaction 934 starts with a MAIL command that gives the sender identification. (In 935 general, the MAIL command may be sent only when no mail transaction 936 is in progress; see Section 4.1.4.) A series of one or more RCPT 937 commands follows, giving the receiver information. Then, a DATA 938 command initiates transfer of the mail data and is terminated by the 939 "end of mail" data indicator, which also confirms the transaction. 941 Mail transactions are also terminated by the RSET command 942 (Section 4.1.1.5), the sending of an EHLO command (Section 3.2), or 943 the sending of a QUIT command (Section 3.8) which terminates both any 944 active mail transaction and the SMTP connection. 946 The first step in the procedure is the MAIL command. 948 MAIL FROM: [SP ] 950 This command tells the SMTP-receiver that a new mail transaction is 951 starting and to reset all its state tables and buffers, including any 952 recipients or mail data. The portion of the first or 953 only argument contains the source mailbox (between "<" and ">" 954 brackets), which can be used to report errors (see Section 4.2 for a 955 discussion of error reporting). If accepted, the SMTP server returns 956 a "250 OK" reply. If the mailbox specification is not acceptable for 957 some reason, the server MUST return a reply indicating whether the 958 failure is permanent (i.e., will occur again if the client tries to 959 send the same address again) or temporary (i.e., the address might be 960 accepted if the client tries again later). Despite the apparent 961 scope of this requirement, there are circumstances in which the 962 acceptability of the reverse-path may not be determined until one or 963 more forward-paths (in RCPT commands) can be examined. In those 964 cases, the server MAY reasonably accept the reverse-path (with a 250 965 reply) and then report problems after the forward-paths are received 966 and examined. Normally, failures produce 550 or 553 replies. 968 Historically, the was permitted to contain more than 969 just a mailbox; however, contemporary systems SHOULD NOT use source 970 routing (see Appendix C). 972 The optional are associated with negotiated SMTP 973 service extensions (see Section 2.2). 975 The second step in the procedure is the RCPT command. This step of 976 the procedure can be repeated any number of times. 978 RCPT TO: [ SP ] 980 The first or only argument to this command includes a forward-path 981 (normally a mailbox and domain, always surrounded by "<" and ">" 982 brackets) identifying one recipient. If accepted, the SMTP server 983 returns a "250 OK" reply and stores the forward-path. If the 984 recipient is known not to be a deliverable address, the SMTP server 985 returns a 550 reply, typically with a string such as "no such user - 986 " and the mailbox name (other circumstances and reply codes are 987 possible). 989 The can contain more than just a mailbox. 990 Historically, the was permitted to contain a source 991 routing list of hosts and the destination mailbox; however, 992 contemporary SMTP clients SHOULD NOT utilize source routes (see 993 Appendix C). Servers MUST be prepared to encounter a list of source 994 routes in the forward-path, but they SHOULD ignore the routes or MAY 995 decline to support the relaying they imply. Similarly, servers MAY 996 decline to accept mail that is destined for other hosts or systems. 997 These restrictions make a server useless as a relay for clients that 998 do not support full SMTP functionality. Consequently, restricted- 999 capability clients MUST NOT assume that any SMTP server on the 1000 Internet can be used as their mail processing (relaying) site. If a 1001 RCPT command appears without a previous MAIL command, the server MUST 1002 return a 503 "Bad sequence of commands" response. The optional 1003 are associated with negotiated SMTP service 1004 extensions (see Section 2.2). [[CREF8: [5321bis] JcK Note for 1005 2821ter (5321bis): this section would be improved by being more 1006 specific about where mail transactions begin and end and then talking 1007 about "transaction state" here, rather than specific prior commands. 1008 --JcK]] 1010 Since it has been a common source of errors, it is worth noting that 1011 spaces are not permitted on either side of the colon following FROM 1012 in the MAIL command or TO in the RCPT command. The syntax is exactly 1013 as given above. 1015 The third step in the procedure is the DATA command (or some 1016 alternative specified in a service extension). 1018 DATA 1020 If accepted, the SMTP server returns a 354 Intermediate reply and 1021 considers all succeeding lines up to but not including the end of 1022 mail data indicator to be the message text. When the end of text is 1023 successfully received and stored, the SMTP-receiver sends a "250 OK" 1024 reply. 1026 Since the mail data is sent on the transmission channel, the end of 1027 mail data must be indicated so that the command and reply dialog can 1028 be resumed. SMTP indicates the end of the mail data by sending a 1029 line containing only a "." (period or full stop). A transparency 1030 procedure is used to prevent this from interfering with the user's 1031 text (see Section 4.5.2). 1033 The end of mail data indicator also confirms the mail transaction and 1034 tells the SMTP server to now process the stored recipients and mail 1035 data. If accepted, the SMTP server returns a "250 OK" reply. The 1036 DATA command can fail at only two points in the protocol exchange: 1038 If there was no MAIL, or no RCPT, command, or all such commands were 1039 rejected, the server MAY return a "command out of sequence" (503) or 1040 "no valid recipients" (554) reply in response to the DATA command. 1041 If one of those replies (or any other 5yz reply) is received, the 1042 client MUST NOT send the message data; more generally, message data 1043 MUST NOT be sent unless a 354 reply is received. 1045 If the verb is initially accepted and the 354 reply issued, the DATA 1046 command should fail only if the mail transaction was incomplete (for 1047 example, no recipients), if resources were unavailable (including, of 1048 course, the server unexpectedly becoming unavailable), or if the 1049 server determines that the message should be rejected for policy or 1050 other reasons. 1052 However, in practice, some servers do not perform recipient 1053 verification until after the message text is received. These servers 1054 SHOULD treat a failure for one or more recipients as a "subsequent 1055 failure" and return a mail message as discussed in Section 6 and, in 1056 particular, in Section 6.1. Using a "550 mailbox not found" (or 1057 equivalent) reply code after the data are accepted makes it difficult 1058 or impossible for the client to determine which recipients failed. 1060 When the RFC 822 format ([12], [11]) is being used, the mail data 1061 include the header fields such as those named Date, Subject, To, Cc, 1062 and From. Server SMTP systems SHOULD NOT reject messages based on 1063 perceived defects in the RFC 822 or MIME (RFC 2045 [24]) message 1064 header section or message body. In particular, they MUST NOT reject 1065 messages in which the numbers of Resent-header fields do not match or 1066 Resent-to appears without Resent-from and/or Resent-date. 1068 Mail transaction commands MUST be used in the order discussed above. 1070 3.4. Forwarding for Address Correction or Updating 1072 Forwarding support is most often required to consolidate and simplify 1073 addresses within, or relative to, some enterprise and less frequently 1074 to establish addresses to link a person's prior address with a 1075 current one. Silent forwarding of messages (without server 1076 notification to the sender), for security or non-disclosure purposes, 1077 is common in the contemporary Internet. 1079 In both the enterprise and the "new address" cases, information 1080 hiding (and sometimes security) considerations argue against exposure 1081 of the "final" address through the SMTP protocol as a side effect of 1082 the forwarding activity. This may be especially important when the 1083 final address may not even be reachable by the sender. Consequently, 1084 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1085 especially the 251 (corrected destination) and 551 reply codes from 1086 RCPT must be evaluated carefully by implementers and, when they are 1087 available, by those configuring systems (see also Section 7.4). 1089 In particular: 1091 o Servers MAY forward messages when they are aware of an address 1092 change. When they do so, they MAY either provide address-updating 1093 information with a 251 code, or may forward "silently" and return 1094 a 250 code. However, if a 251 code is used, they MUST NOT assume 1095 that the client will actually update address information or even 1096 return that information to the user. 1098 Alternately, 1099 o Servers MAY reject messages or return them as non-deliverable when 1100 they cannot be delivered precisely as addressed. When they do so, 1101 they MAY either provide address-updating information with a 551 1102 code, or may reject the message as undeliverable with a 550 code 1103 and no address-specific information. However, if a 551 code is 1104 used, they MUST NOT assume that the client will actually update 1105 address information or even return that information to the user. 1107 SMTP server implementations that support the 251 and/or 551 reply 1108 codes SHOULD provide configuration mechanisms so that sites that 1109 conclude that they would undesirably disclose information can disable 1110 or restrict their use. 1112 3.5. Commands for Debugging Addresses 1114 3.5.1. Overview 1116 SMTP provides commands to verify a user name or obtain the content of 1117 a mailing list. This is done with the VRFY and EXPN commands, which 1118 have character string arguments. Implementations SHOULD support VRFY 1119 and EXPN (however, see Section 3.5.2 and Section 7.3). 1121 For the VRFY command, the string is a user name or a user name and 1122 domain (see below). If a normal (i.e., 250) response is returned, 1123 the response MAY include the full name of the user and MUST include 1124 the mailbox of the user. It MUST be in either of the following 1125 forms: 1127 User Name 1128 local-part@domain 1130 When a name that is the argument to VRFY could identify more than one 1131 mailbox, the server MAY either note the ambiguity or identify the 1132 alternatives. In other words, any of the following are legitimate 1133 responses to VRFY: 1135 553 User ambiguous 1137 or 1139 553- Ambiguous; Possibilities are 1140 553-Joe Smith 1141 553-Harry Smith 1142 553 Melvin Smith 1144 or 1146 553-Ambiguous; Possibilities 1147 553- 1148 553- 1149 553 1151 Under normal circumstances, a client receiving a 553 reply would be 1152 expected to expose the result to the user. Use of exactly the forms 1153 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1154 supplemented by extended reply codes, such as those described in RFC 1155 3463 [34], will facilitate automated translation into other languages 1156 as needed. Of course, a client that was highly automated or that was 1157 operating in another language than English might choose to try to 1158 translate the response to return some other indication to the user 1159 than the literal text of the reply, or to take some automated action 1160 such as consulting a directory service for additional information 1161 before reporting to the user. 1163 For the EXPN command, the string identifies a mailing list, and the 1164 successful (i.e., 250) multiline response MAY include the full name 1165 of the users and MUST give the mailboxes on the mailing list. 1167 In some hosts, the distinction between a mailing list and an alias 1168 for a single mailbox is a bit fuzzy, since a common data structure 1169 may hold both types of entries, and it is possible to have mailing 1170 lists containing only one mailbox. If a request is made to apply 1171 VRFY to a mailing list, a positive response MAY be given if a message 1172 so addressed would be delivered to everyone on the list, otherwise an 1173 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1174 user" or "252 Unable to verify members of mailing list"). If a 1175 request is made to expand a user name, the server MAY return a 1176 positive response consisting of a list containing one name, or an 1177 error MAY be reported (e.g., "550 That is a user name, not a mailing 1178 list"). 1180 In the case of a successful multiline reply (normal for EXPN), 1181 exactly one mailbox is to be specified on each line of the reply. 1182 The case of an ambiguous request is discussed above. 1184 "User name" is a fuzzy term and has been used deliberately. An 1185 implementation of the VRFY or EXPN commands MUST include at least 1186 recognition of local mailboxes as "user names". However, since 1187 current Internet practice often results in a single host handling 1188 mail for multiple domains, hosts, especially hosts that provide this 1189 functionality, SHOULD accept the "local-part@domain" form as a "user 1190 name"; hosts MAY also choose to recognize other strings as "user 1191 names". 1193 The case of expanding a mailbox list requires a multiline reply, such 1194 as: 1196 C: EXPN Example-People 1197 S: 250-Jon Postel 1198 S: 250-Fred Fonebone 1199 S: 250 Sam Q. Smith 1201 or 1203 C: EXPN Executive-Washroom-List 1204 S: 550 Access Denied to You. 1206 The character string arguments of the VRFY and EXPN commands cannot 1207 be further restricted due to the variety of implementations of the 1208 user name and mailbox list concepts. On some systems, it may be 1209 appropriate for the argument of the EXPN command to be a file name 1210 for a file containing a mailing list, but again there are a variety 1211 of file naming conventions in the Internet. Similarly, historical 1212 variations in what is returned by these commands are such that the 1213 response SHOULD be interpreted very carefully, if at all, and SHOULD 1214 generally only be used for diagnostic purposes. 1216 3.5.2. VRFY Normal Response 1218 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1219 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1221 domain name. In circumstances exceptional enough to justify 1222 violating the intent of this specification, free-form text MAY be 1223 returned. In order to facilitate parsing by both computers and 1224 people, addresses SHOULD appear in pointed brackets. When addresses, 1225 rather than free-form debugging information, are returned, EXPN and 1226 VRFY MUST return only valid domain addresses that are usable in SMTP 1227 RCPT commands. Consequently, if an address implies delivery to a 1228 program or other system, the mailbox name used to reach that target 1229 MUST be given. Paths (explicit source routes) MUST NOT be returned 1230 by VRFY or EXPN. 1232 Server implementations SHOULD support both VRFY and EXPN. For 1233 security reasons, implementations MAY provide local installations a 1234 way to disable either or both of these commands through configuration 1235 options or the equivalent (see Section 7.3). When these commands are 1236 supported, they are not required to work across relays when relaying 1237 is supported. Since they were both optional in RFC 821, but VRFY was 1238 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1239 listed as a service extension in an EHLO response. VRFY MAY be 1240 listed as a convenience but, since support for it is required, SMTP 1241 clients are not required to check for its presence on the extension 1242 list before using it. 1244 3.5.3. Meaning of VRFY or EXPN Success Response 1246 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1247 command unless it has actually verified the address. In particular, 1248 a server MUST NOT return 250 if all it has done is to verify that the 1249 syntax given is valid. In that case, 502 (Command not implemented) 1250 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1251 stated elsewhere, implementation (in the sense of actually validating 1252 addresses and returning information) of VRFY and EXPN are strongly 1253 recommended. Hence, implementations that return 500 or 502 for VRFY 1254 are not in full compliance with this specification. 1256 There may be circumstances where an address appears to be valid but 1257 cannot reasonably be verified in real time, particularly when a 1258 server is acting as a mail exchanger for another server or domain. 1259 "Apparent validity", in this case, would normally involve at least 1260 syntax checking and might involve verification that any domains 1261 specified were ones to which the host expected to be able to relay 1262 mail. In these situations, reply code 252 SHOULD be returned. These 1263 cases parallel the discussion of RCPT verification in Section 2.1. 1264 Similarly, the discussion in Section 3.4 applies to the use of reply 1265 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are 1266 recognized but that would be forwarded or rejected were mail received 1267 for them. Implementations generally SHOULD be more aggressive about 1268 address verification in the case of VRFY than in the case of RCPT, 1269 even if it takes a little longer to do so. 1271 3.5.4. Semantics and Applications of EXPN 1273 EXPN is often very useful in debugging and understanding problems 1274 with mailing lists and multiple-target-address aliases. Some systems 1275 have attempted to use source expansion of mailing lists as a means of 1276 eliminating duplicates. The propagation of aliasing systems with 1277 mail on the Internet for hosts (typically with MX and CNAME DNS 1278 records), for mailboxes (various types of local host aliases), and in 1279 various proxying arrangements has made it nearly impossible for these 1280 strategies to work consistently, and mail systems SHOULD NOT attempt 1281 them. 1283 3.6. Relaying and Mail Routing 1285 3.6.1. Source Routes and Relaying 1287 In general, the availability of Mail eXchanger records in the domain 1288 name system (RFC 1035 [4], RFC 974 [15]) makes the use of explicit 1289 source routes in the Internet mail system unnecessary. Many 1290 historical problems with the interpretation of explicit source routes 1291 have made their use undesirable. SMTP clients SHOULD NOT generate 1292 explicit source routes except under unusual circumstances. SMTP 1293 servers MAY decline to act as mail relays or to accept addresses that 1294 specify source routes. When route information is encountered, SMTP 1295 servers MAY ignore the route information and simply send to the final 1296 destination specified as the last element in the route and SHOULD do 1297 so. There has been an invalid practice of using names that do not 1298 appear in the DNS as destination names, with the senders counting on 1299 the intermediate hosts specified in source routing to resolve any 1300 problems. If source routes are stripped, this practice will cause 1301 failures. This is one of several reasons why SMTP clients MUST NOT 1302 generate invalid source routes or depend on serial resolution of 1303 names in such routes. [[CREF9: [5321bis] Jck 20091023: "of names..." 1304 added for clarity"]] 1306 When source routes are not used, the process described in RFC 821 for 1307 constructing a reverse-path from the forward-path is not applicable 1308 and the reverse-path at the time of delivery will simply be the 1309 address that appeared in the MAIL command. 1311 3.6.2. Mail eXchange Records and Relaying 1313 A relay SMTP server is usually the target of a DNS MX record that 1314 designates it, rather than the final delivery system. The relay 1315 server may accept or reject the task of relaying the mail in the same 1316 way it accepts or rejects mail for a local user. If it accepts the 1317 task, it then becomes an SMTP client, establishes a transmission 1318 channel to the next SMTP server specified in the DNS (according to 1319 the rules in Section 5), and sends it the mail. If it declines to 1320 relay mail to a particular address for policy reasons, a 550 response 1321 SHOULD be returned. 1323 This specification does not deal with the verification of return 1324 paths for use in delivery notifications. Recent work, such as that 1325 on SPF [41] and DKIM [43] [44], has been done to provide ways to 1326 ascertain that an address is valid or belongs to the person who 1327 actually sent the message. 1328 [[5321bis Editor's Note: Proposed erratum (4055) suggests that DKIM 1329 and SPF have nothing to do with this and that everything after the 1330 first sentence should be dropped. An alternative would be to tune 1331 the texts. ???]] 1332 A server MAY attempt to verify the return path before using its 1333 address for delivery notifications, but methods of doing so are not 1334 defined here nor is any particular method recommended at this time. 1336 3.6.3. Message Submission Servers as Relays 1338 Many mail-sending clients exist, especially in conjunction with 1339 facilities that receive mail via POP3 or IMAP, that have limited 1340 capability to support some of the requirements of this specification, 1341 such as the ability to queue messages for subsequent delivery 1342 attempts. For these clients, it is common practice to make private 1343 arrangements to send all messages to a single server for processing 1344 and subsequent distribution. SMTP, as specified here, is not ideally 1345 suited for this role. A standardized mail submission protocol has 1346 been developed that is gradually superseding practices based on SMTP 1347 (see RFC 4409 [42]). In any event, because these arrangements are 1348 private and fall outside the scope of this specification, they are 1349 not described here. 1351 It is important to note that MX records can point to SMTP servers 1352 that act as gateways into other environments, not just SMTP relays 1353 and final delivery systems; see Sections 3.7 and 5. 1355 If an SMTP server has accepted the task of relaying the mail and 1356 later finds that the destination is incorrect or that the mail cannot 1357 be delivered for some other reason, then it MUST construct an 1358 "undeliverable mail" notification message and send it to the 1359 originator of the undeliverable mail (as indicated by the reverse- 1360 path). Formats specified for non-delivery reports by other standards 1361 (see, for example, RFC 3461 [33] and RFC 3464 [35]) SHOULD be used if 1362 possible. 1364 This notification message must be from the SMTP server at the relay 1365 host or the host that first determines that delivery cannot be 1366 accomplished. Of course, SMTP servers MUST NOT send notification 1367 messages about problems transporting notification messages. One way 1368 to prevent loops in error reporting is to specify a null reverse-path 1369 in the MAIL command of a notification message. When such a message 1370 is transmitted, the reverse-path MUST be set to null (see 1371 Section 4.5.5 for additional discussion). A MAIL command with a null 1372 reverse-path appears as follows: 1374 MAIL FROM:<> 1376 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1377 act upon the header section or body of the message data and MUST NOT 1378 do so except to add its own "Received:" header field (Section 4.4) 1379 and, optionally, to attempt to detect looping in the mail system (see 1380 Section 6.3). Of course, this prohibition also applies to any 1381 modifications of these header fields or text (see also Section 7.9). 1383 3.7. Mail Gatewaying 1385 While the relay function discussed above operates within the Internet 1386 SMTP transport service environment, MX records or various forms of 1387 explicit routing may require that an intermediate SMTP server perform 1388 a translation function between one transport service and another. As 1389 discussed in Section 2.3.10, when such a system is at the boundary 1390 between two transport service environments, we refer to it as a 1391 "gateway" or "gateway SMTP". 1393 Gatewaying mail between different mail environments, such as 1394 different mail formats and protocols, is complex and does not easily 1395 yield to standardization. However, some general requirements may be 1396 given for a gateway between the Internet and another mail 1397 environment. 1399 3.7.1. Header Fields in Gatewaying 1401 Header fields MAY be rewritten when necessary as messages are 1402 gatewayed across mail environment boundaries. This may involve 1403 inspecting the message body or interpreting the local-part of the 1404 destination address in spite of the prohibitions in Section 6.4. 1406 Other mail systems gatewayed to the Internet often use a subset of 1407 the RFC 822 header section or provide similar functionality with a 1408 different syntax, but some of these mail systems do not have an 1409 equivalent to the SMTP envelope. Therefore, when a message leaves 1410 the Internet environment, it may be necessary to fold the SMTP 1411 envelope information into the message header section. A possible 1412 solution would be to create new header fields to carry the envelope 1413 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1414 would require changes in mail programs in foreign environments and 1415 might risk disclosure of private information (see Section 7.2). 1417 3.7.2. Received Lines in Gatewaying 1419 When forwarding a message into or out of the Internet environment, a 1420 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1421 way a Received: line that is already in the header section. 1423 "Received:" header fields of messages originating from other 1424 environments may not conform exactly to this specification. However, 1425 the most important use of Received: lines is for debugging mail 1426 faults, and this debugging can be severely hampered by well-meaning 1427 gateways that try to "fix" a Received: line. As another consequence 1428 of trace header fields arising in non-SMTP environments, receiving 1429 systems MUST NOT reject mail based on the format of a trace header 1430 field and SHOULD be extremely robust in the light of unexpected 1431 information or formats in those header fields. 1433 The gateway SHOULD indicate the environment and protocol in the "via" 1434 clauses of Received header field(s) that it supplies. 1436 3.7.3. Addresses in Gatewaying 1438 From the Internet side, the gateway SHOULD accept all valid address 1439 formats in SMTP commands and in the RFC 822 header section, and all 1440 valid RFC 822 messages. Addresses and header fields generated by 1441 gateways MUST conform to applicable standards (including this one and 1442 RFC 5322 [11]). Gateways are, of course, subject to the same rules 1443 for handling source routes as those described for other SMTP systems 1444 in Section 3.3. 1446 3.7.4. Other Header Fields in Gatewaying 1448 The gateway MUST ensure that all header fields of a message that it 1449 forwards into the Internet mail environment meet the requirements for 1450 Internet mail. In particular, all addresses in "From:", "To:", 1451 "Cc:", etc., header fields MUST be transformed (if necessary) to 1452 satisfy the standard header syntax of RFC 5322 [11], MUST reference 1453 only fully-qualified domain names, and MUST be effective and useful 1454 for sending replies. The translation algorithm used to convert mail 1455 from the Internet protocols to another environment's protocol SHOULD 1456 ensure that error messages from the foreign mail environment are 1457 delivered to the reverse-path from the SMTP envelope, not to an 1458 address in the "From:", "Sender:", or similar header fields of the 1459 message. 1461 3.7.5. Envelopes in Gatewaying 1463 Similarly, when forwarding a message from another environment into 1464 the Internet, the gateway SHOULD set the envelope return path in 1465 accordance with an error message return address, if supplied by the 1466 foreign environment. If the foreign environment has no equivalent 1467 concept, the gateway must select and use a best approximation, with 1468 the message originator's address as the default of last resort. 1470 3.8. Terminating Sessions and Connections 1472 An SMTP connection is terminated when the client sends a QUIT 1473 command. The server responds with a positive reply code, after which 1474 it closes the connection. 1476 An SMTP server MUST NOT intentionally close the connection under 1477 normal operational circumstances (see Section 7.8) except: 1479 o After receiving a QUIT command and responding with a 221 reply. 1481 o After detecting the need to shut down the SMTP service and 1482 returning a 421 reply code. This reply code can be issued after 1483 the server receives any command or, if necessary, asynchronously 1484 from command receipt (on the assumption that the client will 1485 receive it after the next command is issued). 1487 o After a timeout, as specified in Section 4.5.3.2, occurs waiting 1488 for the client to send a command or data. 1490 In particular, a server that closes connections in response to 1491 commands that are not understood is in violation of this 1492 specification. Servers are expected to be tolerant of unknown 1493 commands, issuing a 500 reply and awaiting further instructions from 1494 the client. 1496 An SMTP server that is forcibly shut down via external means SHOULD 1497 attempt to send a line containing a 421 reply code to the SMTP client 1498 before exiting. The SMTP client will normally read the 421 reply 1499 code after sending its next command. 1501 SMTP clients that experience a connection close, reset, or other 1502 communications failure due to circumstances not under their control 1503 (in violation of the intent of this specification but sometimes 1504 unavoidable) SHOULD, to maintain the robustness of the mail system, 1505 treat the mail transaction as if a 421 response had been received and 1506 act accordingly. 1508 3.9. Mailing Lists and Aliases 1510 [[CREF10: [5321bis] If "alias and list models" are explained 1511 elsewhere, cross reference". Also note that this section appears to 1512 prohibit an exploder from adding List-* headers. That needs to be 1513 finessed.]] 1514 An SMTP-capable host SHOULD support both the alias and the list 1515 models of address expansion for multiple delivery. When a message is 1516 delivered or forwarded to each address of an expanded list form, the 1517 return address in the envelope ("MAIL FROM:") MUST be changed to be 1518 the address of a person or other entity who administers the list. 1519 However, in this case, the message header section (RFC 5322 [11]) 1520 MUST be left unchanged; in particular, the "From" field of the header 1521 section is unaffected. 1523 An important mail facility is a mechanism for multi-destination 1524 delivery of a single message, by transforming (or "expanding" or 1525 "exploding") a pseudo-mailbox address into a list of destination 1526 mailbox addresses. When a message is sent to such a pseudo-mailbox 1527 (sometimes called an "exploder"), copies are forwarded or 1528 redistributed to each mailbox in the expanded list. Servers SHOULD 1529 simply utilize the addresses on the list; application of heuristics 1530 or other matching rules to eliminate some addresses, such as that of 1531 the originator, is strongly discouraged. We classify such a pseudo- 1532 mailbox as an "alias" or a "list", depending upon the expansion 1533 rules. 1535 3.9.1. Alias 1537 To expand an alias, the recipient mailer simply replaces the pseudo- 1538 mailbox address in the envelope with each of the expanded addresses 1539 in turn; the rest of the envelope and the message body are left 1540 unchanged. The message is then delivered or forwarded to each 1541 expanded address. 1543 3.9.2. List 1545 A mailing list may be said to operate by "redistribution" rather than 1546 by "forwarding". To expand a list, the recipient mailer replaces the 1547 pseudo-mailbox address in the envelope with each of the expanded 1548 addresses in turn. The return (backward-pointing) address in the 1549 envelope is changed so that all error messages generated by the final 1550 deliveries will be returned to a list administrator, not to the 1551 message originator, who generally has no control over the contents of 1552 the list and will typically find error messages annoying. Note that 1553 the key difference between handling aliases (Section 3.9.1) and 1554 forwarding (this subsection) is the change to the backward-pointing 1555 address in this case. When a list constrains its processing to the 1556 very limited set of modifications and actions described here, it is 1557 attempting to emulate an MTA; such lists can be treated as a 1558 continuation in email transit. 1560 There exist mailing lists that perform additional, sometimes 1561 extensive, modifications to a message and its envelope. Such mailing 1562 lists need to be viewed as full MUAs, which accept a delivery and 1563 post a new message. 1565 4. The SMTP Specifications 1567 4.1. SMTP Commands 1569 4.1.1. Command Semantics and Syntax 1571 The SMTP commands define the mail transfer or the mail system 1572 function requested by the user. SMTP commands are character strings 1573 terminated by . The commands themselves are alphabetic 1574 characters terminated by if parameters follow and 1575 otherwise. (In the interest of improved interoperability, SMTP 1576 receivers SHOULD tolerate trailing white space before the terminating 1577 .) The syntax of the local part of a mailbox MUST conform to 1578 receiver site conventions and the syntax specified in Section 4.1.2. 1579 The SMTP commands are discussed below. The SMTP replies are 1580 discussed in Section 4.2. 1582 A mail transaction involves several data objects that are 1583 communicated as arguments to different commands. The reverse-path is 1584 the argument of the MAIL command, the forward-path is the argument of 1585 the RCPT command, and the mail data is the argument of the DATA 1586 command. These arguments or data objects must be transmitted and 1587 held, pending the confirmation communicated by the end of mail data 1588 indication that finalizes the transaction. The model for this is 1589 that distinct buffers are provided to hold the types of data objects; 1590 that is, there is a reverse-path buffer, a forward-path buffer, and a 1591 mail data buffer. Specific commands cause information to be appended 1592 to a specific buffer, or cause one or more buffers to be cleared. 1594 Several commands (RSET, DATA, QUIT) are specified as not permitting 1595 parameters. In the absence of specific extensions offered by the 1596 server and accepted by the client, clients MUST NOT send such 1597 parameters and servers SHOULD reject commands containing them as 1598 having invalid syntax. 1600 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1602 These commands are used to identify the SMTP client to the SMTP 1603 server. The argument clause contains the fully-qualified domain name 1604 of the SMTP client, if one is available. In situations in which the 1605 SMTP client system does not have a meaningful domain name (e.g., when 1606 its address is dynamically allocated and no reverse mapping record is 1607 available), the client SHOULD send an address literal (see 1608 Section 4.1.3). 1610 RFC 2821, and some earlier informal practices, encouraged following 1611 the literal by information that would help to identify the client 1612 system. That convention was not widely supported, and many SMTP 1613 servers considered it an error. In the interest of interoperability, 1614 it is probably wise for servers to be prepared for this string to 1615 occur, but SMTP clients SHOULD NOT send it. 1617 The SMTP server identifies itself to the SMTP client in the 1618 connection greeting reply and in the response to this command. 1620 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1621 command. If the SMTP server supports the SMTP service extensions, it 1622 will give a successful response, a failure response, or an error 1623 response. If the SMTP server, in violation of this specification, 1624 does not support any SMTP service extensions, it will generate an 1625 error response. Older client SMTP systems MAY, as discussed above, 1626 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1627 support the HELO command and reply properly to it. In any event, a 1628 client MUST issue HELO or EHLO before starting a mail transaction. 1630 These commands, and a "250 OK" reply to one of them, confirm that 1631 both the SMTP client and the SMTP server are in the initial state, 1632 that is, there is no transaction in progress and all state tables and 1633 buffers are cleared. 1635 Syntax: 1637 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1639 helo = "HELO" SP Domain CRLF 1641 Normally, the response to EHLO will be a multiline reply. Each line 1642 of the response contains a keyword and, optionally, one or more 1643 parameters. Following the normal syntax for multiline replies, these 1644 keywords follow the code (250) and a hyphen for all but the last 1645 line, and the code and a space for the last line. The syntax for a 1646 positive response, using the ABNF notation and terminal symbols of 1647 RFC 5234 [10], is: 1649 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1650 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1651 *( "250-" ehlo-line CRLF ) 1652 "250" SP ehlo-line CRLF ) 1654 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1655 ; string of any characters other than CR or LF 1657 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1659 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1660 ; additional syntax of ehlo-params depends on 1661 ; ehlo-keyword 1663 ehlo-param = 1*(%d33-126) 1664 ; any CHAR excluding and all 1665 ; control characters (US-ASCII 0-31 and 127 1666 ; inclusive) 1668 Although EHLO keywords may be specified in upper, lower, or mixed 1669 case, they MUST always be recognized and processed in a case- 1670 insensitive manner. This is simply an extension of practices 1671 specified in RFC 821 and Section 2.4. 1673 The EHLO response MUST contain keywords (and associated parameters if 1674 required) for all commands not listed as "required" in Section 4.5.1 1675 excepting only private-use commands as described in Section 4.1.5. 1676 Private-use commands MAY be listed. 1678 4.1.1.2. MAIL (MAIL) 1680 This command is used to initiate a mail transaction in which the mail 1681 data is delivered to an SMTP server that may, in turn, deliver it to 1682 one or more mailboxes or pass it on to another system (possibly using 1683 SMTP). The argument clause contains a reverse-path and may contain 1684 optional parameters. In general, the MAIL command may be sent only 1685 when no mail transaction is in progress, see Section 4.1.4. 1687 The reverse-path consists of the sender mailbox. Historically, that 1688 mailbox might optionally have been preceded by a list of hosts, but 1689 that behavior is now deprecated (see Appendix C). In some types of 1690 reporting messages for which a reply is likely to cause a mail loop 1691 (for example, mail delivery and non-delivery notifications), the 1692 reverse-path may be null (see Section 3.6). 1694 This command clears the reverse-path buffer, the forward-path buffer, 1695 and the mail data buffer, and it inserts the reverse-path information 1696 from its argument clause into the reverse-path buffer. 1698 If service extensions were negotiated, the MAIL command may also 1699 carry parameters associated with a particular service extension. 1701 Syntax: 1703 mail = "MAIL FROM:" Reverse-path 1704 [SP Mail-parameters] CRLF 1706 4.1.1.3. RECIPIENT (RCPT) 1708 This command is used to identify an individual recipient of the mail 1709 data; multiple recipients are specified by multiple uses of this 1710 command. The argument clause contains a forward-path and may contain 1711 optional parameters. 1713 The forward-path normally consists of the required destination 1714 mailbox. Sending systems SHOULD NOT generate the optional list of 1715 hosts known as a source route. Receiving systems MUST recognize 1716 source route syntax but SHOULD strip off the source route 1717 specification and utilize the domain name associated with the mailbox 1718 as if the source route had not been provided. 1720 Similarly, relay hosts SHOULD strip or ignore source routes, and 1721 names MUST NOT be copied into the reverse-path. When mail reaches 1722 its ultimate destination (the forward-path contains only a 1723 destination mailbox), the SMTP server inserts it into the destination 1724 mailbox in accordance with its host mail conventions. 1726 This command appends its forward-path argument to the forward-path 1727 buffer; it does not change the reverse-path buffer nor the mail data 1728 buffer. 1730 For example, mail received at relay host xyz.com with envelope 1731 commands 1733 MAIL FROM: 1734 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1736 will normally be sent directly on to host d.bar.org with envelope 1737 commands 1739 MAIL FROM: 1740 RCPT TO: 1742 As provided in Appendix C, xyz.com MAY also choose to relay the 1743 message to hosta.int, using the envelope commands 1745 MAIL FROM: 1746 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1748 or to jkl.org, using the envelope commands 1750 MAIL FROM: 1751 RCPT TO:<@jkl.org:userc@d.bar.org> 1753 Attempting to use relaying this way is now strongly discouraged. 1754 Since hosts are not required to relay mail at all, xyz.com MAY also 1755 reject the message entirely when the RCPT command is received, using 1756 a 550 code (since this is a "policy reason"). 1758 If service extensions were negotiated, the RCPT command may also 1759 carry parameters associated with a particular service extension 1760 offered by the server. The client MUST NOT transmit parameters other 1761 than those associated with a service extension offered by the server 1762 in its EHLO response. 1764 Syntax: 1766 rcpt = "RCPT TO:" ( "" / "" / 1767 Forward-path ) [SP Rcpt-parameters] CRLF 1769 Note that, in a departure from the usual rules for 1770 local-parts, the "Postmaster" string shown above is 1771 treated as case-insensitive. 1773 4.1.1.4. DATA (DATA) 1775 The receiver normally sends a 354 response to DATA, and then treats 1776 the lines (strings ending in sequences, as described in 1777 Section 2.3.7) following the command as mail data from the sender. 1778 This command causes the mail data to be appended to the mail data 1779 buffer. The mail data may contain any of the 128 ASCII character 1780 codes, although experience has indicated that use of control 1781 characters other than SP, HT, CR, and LF may cause problems and 1782 SHOULD be avoided when possible. 1784 The mail data are terminated by a line containing only a period, that 1785 is, the character sequence ".", where the first is 1786 actually the terminator of the previous line (see Section 4.5.2). 1787 This is the end of mail data indication. The first of this 1788 terminating sequence is also the that ends the final line of 1789 the data (message text) or, if there was no mail data, ends the DATA 1790 command itself (the "no mail data" case does not conform to this 1791 specification since it would require that neither the trace header 1792 fields required by this specification nor the message header section 1793 required by RFC 5322 [11] be transmitted). An extra MUST NOT 1794 be added, as that would cause an empty line to be added to the 1795 message. The only exception to this rule would arise if the message 1796 body were passed to the originating SMTP-sender with a final "line" 1797 that did not end in ; in that case, the originating SMTP system 1798 MUST either reject the message as invalid or add in order to 1799 have the receiving SMTP server recognize the "end of data" condition. 1801 The custom of accepting lines ending only in , as a concession to 1802 non-conforming behavior on the part of some UNIX systems, has proven 1803 to cause more interoperability problems than it solves, and SMTP 1804 server systems MUST NOT do this, even in the name of improved 1805 robustness. In particular, the sequence "." (bare line 1806 feeds, without carriage returns) MUST NOT be treated as equivalent to 1807 . as the end of mail data indication. 1809 Receipt of the end of mail data indication requires the server to 1810 process the stored mail transaction information. This processing 1811 consumes the information in the reverse-path buffer, the forward-path 1812 buffer, and the mail data buffer, and on the completion of this 1813 command these buffers are cleared. If the processing is successful, 1814 the receiver MUST send an OK reply. If the processing fails, the 1815 receiver MUST send a failure reply. The SMTP model does not allow 1816 for partial failures at this point: either the message is accepted by 1817 the server for delivery and a positive response is returned or it is 1818 not accepted and a failure reply is returned. In sending a positive 1819 "250 OK" completion reply to the end of data indication, the receiver 1820 takes full responsibility for the message (see Section 6.1). Errors 1821 that are diagnosed subsequently MUST be reported in a mail message, 1822 as discussed in Section 4.4. 1824 When the SMTP server accepts a message either for relaying or for 1825 final delivery, it inserts a trace record (also referred to 1826 interchangeably as a "time stamp line" or "Received" line) at the top 1827 of the mail data. This trace record indicates the identity of the 1828 host that sent the message, the identity of the host that received 1829 the message (and is inserting this time stamp), and the date and time 1830 the message was received. Relayed messages will have multiple time 1831 stamp lines. Details for formation of these lines, including their 1832 syntax, is specified in Section 4.4. 1834 Additional discussion about the operation of the DATA command appears 1835 in Section 3.3. 1837 Syntax: 1839 data = "DATA" CRLF 1841 4.1.1.5. RESET (RSET) 1843 This command specifies that the current mail transaction will be 1844 aborted. Any stored sender, recipients, and mail data MUST be 1845 discarded, and all buffers and state tables cleared. The receiver 1846 MUST send a "250 OK" reply to a RSET command with no arguments. A 1847 reset command may be issued by the client at any time. It is 1848 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1849 immediately after EHLO, before EHLO is issued in the session, after 1850 an end of data indicator has been sent and acknowledged, or 1851 immediately before a QUIT. An SMTP server MUST NOT close the 1852 connection as the result of receiving a RSET; that action is reserved 1853 for QUIT (see Section 4.1.1.10). 1855 Since EHLO implies some additional processing and response by the 1856 server, RSET will normally be more efficient than reissuing that 1857 command, even though the formal semantics are the same. 1859 There are circumstances, contrary to the intent of this 1860 specification, in which an SMTP server may receive an indication that 1861 the underlying TCP connection has been closed or reset. To preserve 1862 the robustness of the mail system, SMTP servers SHOULD be prepared 1863 for this condition and SHOULD treat it as if a QUIT had been received 1864 before the connection disappeared. 1866 Syntax: 1868 rset = "RSET" CRLF 1870 4.1.1.6. VERIFY (VRFY) 1872 This command asks the receiver to confirm that the argument 1873 identifies a user or mailbox. If it is a user name, information is 1874 returned as specified in Section 3.5. 1876 This command has no effect on the reverse-path buffer, the forward- 1877 path buffer, or the mail data buffer. 1879 Syntax: 1881 vrfy = "VRFY" SP String CRLF 1883 4.1.1.7. EXPAND (EXPN) 1885 This command asks the receiver to confirm that the argument 1886 identifies a mailing list, and if so, to return the membership of 1887 that list. If the command is successful, a reply is returned 1888 containing information as described in Section 3.5. This reply will 1889 have multiple lines except in the trivial case of a one-member list. 1891 This command has no effect on the reverse-path buffer, the forward- 1892 path buffer, or the mail data buffer, and it may be issued at any 1893 time. 1895 Syntax: 1897 expn = "EXPN" SP String CRLF 1899 4.1.1.8. HELP (HELP) 1901 This command causes the server to send helpful information to the 1902 client. The command MAY take an argument (e.g., any command name) 1903 and return more specific information as a response. 1905 This command has no effect on the reverse-path buffer, the forward- 1906 path buffer, or the mail data buffer, and it may be issued at any 1907 time. 1909 SMTP servers SHOULD support HELP without arguments and MAY support it 1910 with arguments. 1912 Syntax: 1914 help = "HELP" [ SP String ] CRLF 1916 4.1.1.9. NOOP (NOOP) 1918 This command does not affect any parameters or previously entered 1919 commands. It specifies no action other than that the receiver send a 1920 "250 OK" reply. 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. If a parameter string is specified, servers SHOULD ignore it. 1926 Syntax: 1928 noop = "NOOP" [ SP String ] CRLF 1930 4.1.1.10. QUIT (QUIT) 1932 This command specifies that the receiver MUST send a "221 OK" reply, 1933 and then close the transmission channel. 1935 The receiver MUST NOT intentionally close the transmission channel 1936 until it receives and replies to a QUIT command (even if there was an 1937 error). The sender MUST NOT intentionally close the transmission 1938 channel until it sends a QUIT command, and it SHOULD wait until it 1939 receives the reply (even if there was an error response to a previous 1940 command). If the connection is closed prematurely due to violations 1941 of the above or system or network failure, the server MUST cancel any 1942 pending transaction, but not undo any previously completed 1943 transaction, and generally MUST act as if the command or transaction 1944 in progress had received a temporary error (i.e., a 4yz response). 1946 The QUIT command may be issued at any time. Any current uncompleted 1947 mail transaction will be aborted. 1949 Syntax: 1951 quit = "QUIT" CRLF 1953 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1955 If the server SMTP does not recognize or cannot implement one or more 1956 of the parameters associated with a particular MAIL FROM or RCPT TO 1957 command, it will return code 555. 1959 If, for some reason, the server is temporarily unable to accommodate 1960 one or more of the parameters associated with a MAIL FROM or RCPT TO 1961 command, and if the definition of the specific parameter does not 1962 mandate the use of another code, it should return code 455. 1964 Errors specific to particular parameters and their values will be 1965 specified in the parameter's defining RFC. 1967 4.1.2. Command Argument Syntax 1969 The syntax of the argument clauses of the above commands (using the 1970 syntax specified in RFC 5234 [10] where applicable) is given below. 1971 Some of the productions given below are used only in conjunction with 1972 source routes as described in Appendix C. Some terminals not defined 1973 in this document, but are defined elsewhere, specifically: 1975 In the "core" syntax in Appendix B of RFC 5234 [10]: ALPHA , CRLF 1976 , DIGIT , HEXDIG , and SP 1978 In the message format syntax in RFC 5322 [11]: atext , CFWS , and 1979 FWS . 1981 Reverse-path = Path / "<>" 1983 Forward-path = Path 1985 Path = "<" [ A-d-l ":" ] Mailbox ">" 1987 A-d-l = At-domain *( "," At-domain ) 1988 ; Note that this form, the so-called "source 1989 ; route", MUST BE accepted, SHOULD NOT be 1990 ; generated, and SHOULD be ignored. 1992 At-domain = "@" Domain 1994 Mail-parameters = esmtp-param *(SP esmtp-param) 1996 Rcpt-parameters = esmtp-param *(SP esmtp-param) 1998 esmtp-param = esmtp-keyword ["=" esmtp-value] 2000 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 2001 esmtp-value = 1*(%d33-60 / %d62-126) 2002 ; any CHAR excluding "=", SP, and control 2003 ; characters. If this string is an email address, 2004 ; i.e., a Mailbox, then the "xtext" syntax [33] 2005 ; SHOULD be used. 2007 Keyword = Ldh-str 2009 Argument = Atom 2011 Domain = sub-domain *("." sub-domain) 2013 sub-domain = Let-dig [Ldh-str] 2015 Let-dig = ALPHA / DIGIT 2017 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2019 address-literal = "[" ( IPv4-address-literal / 2020 IPv6-address-literal / 2021 General-address-literal ) "]" 2022 ; See Section 4.1.3 2024 Mailbox = Local-part "@" ( Domain / address-literal ) 2026 Local-part = Dot-string / Quoted-string 2027 ; MAY be case-sensitive 2029 Dot-string = Atom *("." Atom) 2031 Atom = 1*atext 2033 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2035 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2037 quoted-pairSMTP = %d92 %d32-126 2038 ; i.e., backslash followed by any ASCII 2039 ; graphic (including itself) or SPace 2041 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2042 ; i.e., within a quoted string, any 2043 ; ASCII graphic or space is permitted 2044 ; without backslash-quoting except 2045 ; double-quote and the backslash itself. 2047 String = Atom / Quoted-string 2048 While the above definition for Local-part is relatively permissive, 2049 for maximum interoperability, a host that expects to receive mail 2050 SHOULD avoid defining mailboxes where the Local-part requires (or 2051 uses) the Quoted-string form or where the Local-part is case- 2052 sensitive. For any purposes that require generating or comparing 2053 Local-parts (e.g., to specific mailbox names), all quoted forms MUST 2054 be treated as equivalent, and the sending system SHOULD transmit the 2055 form that uses the minimum quoting possible. 2057 Systems MUST NOT define mailboxes in such a way as to require the use 2058 in SMTP of non-ASCII characters (octets with the high order bit set 2059 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2060 These characters MUST NOT be used in MAIL or RCPT commands or other 2061 commands that require mailbox names. 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 To promote interoperability and consistent with long-standing 2070 guidance about conservative use of the DNS in naming and applications 2071 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2072 characters outside the set of alphabetic characters, digits, and 2073 hyphen MUST NOT appear in domain name labels for SMTP clients or 2074 servers. In particular, the underscore character is not permitted. 2075 SMTP servers that receive a command in which invalid character codes 2076 have been employed, and for which there are no other reasons for 2077 rejection, MUST reject that command with a 501 response (this rule, 2078 like others, could be overridden by appropriate SMTP extensions). 2080 4.1.3. Address Literals 2082 Sometimes a host is not known to the domain name system and 2083 communication (and, in particular, communication to report and repair 2084 the error) is blocked. To bypass this barrier, a special literal 2085 form of the address is allowed as an alternative to a domain name. 2086 For IPv4 addresses, this form uses four small decimal integers 2087 separated by dots and enclosed by brackets such as [123.255.37.2], 2088 which indicates an (IPv4) Internet Address in sequence-of-octets 2089 form. For IPv6 and other forms of addressing that might eventually 2090 be standardized, the form consists of a standardized "tag" that 2091 identifies the address syntax, a colon, and the address itself, in a 2092 format specified as part of the relevant standards (i.e., RFC 4291 2093 [9] for IPv6). 2094 [[CREF11: [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet 2095 another modification to the IPv6 address literal syntax, based on 2096 part on RFC 5952. We should consider whether those, or other, 2097 modifications are appropriate and/or whether, given both the issues 2098 of spam/malware and servers supporting multiple domains, it it time 2099 to deprecate mailboxes containing address literals entirely (EHLO 2100 fields may be a different issue). If we are going to allow IPv6 2101 address literals, it may be time to incorporate something by 2102 reference rather than including specific syntax here (RFC 5952 is 14 2103 pages long and does not contain any ABNF).]] 2105 Specifically: 2107 IPv4-address-literal = Snum 3("." Snum) 2109 IPv6-address-literal = "IPv6:" IPv6-addr 2111 General-address-literal = Standardized-tag ":" 1*dcontent 2113 Standardized-tag = Ldh-str 2114 ; Standardized-tag MUST be specified in a 2115 ; Standards-Track RFC and registered with IANA 2117 dcontent = %d33-90 / ; Printable US-ASCII 2118 %d94-126 ; excl. "[", "\", "]" 2120 Snum = 1*3DIGIT 2121 ; representing a decimal integer 2122 ; value in the range 0 through 255 2124 IPv6-addr = 6( h16 ":" ) ls32 2125 / "::" 5( h16 ":" ) ls32 2126 / [ h16 ] "::" 4( h16 ":" ) ls32 2127 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2128 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2129 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2130 / [ *4( h16 ":" ) h16 ] "::" ls32 2131 / [ *5( h16 ":" ) h16 ] "::" h16 2132 / [ *6( h16 ":" ) h16 ] "::" 2133 ; This definition is consistent with the one for 2134 ; URIs [40]. 2136 ls32 = ( h16 ":" h16 ) / IPv4address 2137 ; least-significant 32 bits of address 2139 h16 = 1*4HEXDIG 2140 ; 16 bits of address represented in hexadecimal 2141 [[CREF12: [5321bis](2821ter) 2821bis Last Call 2142 comment]] 2144 4.1.4. Order of Commands 2146 There are restrictions on the order in which these commands may be 2147 used. 2149 A session that will contain mail transactions MUST first be 2150 initialized by the use of the EHLO command. An SMTP server SHOULD 2151 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2152 without this initialization. 2154 An EHLO command MAY be issued by a client later in the session. If 2155 it is issued after the session begins and the EHLO command is 2156 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2157 and reset the state exactly as if a RSET command had been issued 2158 (specifically, it terminates any mail transaction that was in 2159 progress, see Section 3.3). In other words, the sequence of RSET 2160 followed immediately by EHLO is redundant, but not harmful other than 2161 in the performance cost of executing unnecessary commands. However 2162 the response to an additional EHLO command MAY be different from that 2163 from prior ones; the client MUST rely only on the responses from the 2164 most recent EHLO command. 2166 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2167 502, or 550 failure replies MUST be returned as appropriate. The 2168 SMTP server MUST stay in the same state after transmitting these 2169 replies that it was in before the EHLO was received. 2171 The SMTP client MUST, if possible, ensure that the domain parameter 2172 to the EHLO command is a primary host name as specified for this 2173 command in Section 2.3.5. If this is not possible (e.g., when the 2174 client's address is dynamically assigned and the client does not have 2175 an obvious name), an address literal SHOULD be substituted for the 2176 domain name. 2178 An SMTP server MAY verify that the domain name argument in the EHLO 2179 command actually corresponds to the IP address of the client. 2180 [[CREF13: [5321bis] [[Note in draft -- proposed change to "An SMTP 2181 server MAY verify that the domain name argument in the EHLO command 2182 has an address record matching the IP address of the client." --David 2183 MacQuigg, david_macquigg@yahoo.com, Friday, 20090130 0637 -0700]]]] 2184 However, if the verification fails, the server MUST NOT refuse to 2185 accept a message on that basis. Information captured in the 2186 verification attempt is for logging and tracing purposes. Note that 2187 this prohibition applies to the matching of the parameter to its IP 2188 address only; see Section 7.9 for a more extensive discussion of 2189 rejecting incoming connections or mail messages. 2191 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2192 during a session, or without previously initializing a session. SMTP 2193 servers SHOULD process these normally (that is, not return a 503 2194 code) even if no EHLO command has yet been received; clients SHOULD 2195 open a session with EHLO before sending these commands. 2197 If these rules are followed, the example in RFC 821 that shows "550 2198 access denied to you" in response to an EXPN command is incorrect 2199 unless an EHLO command precedes the EXPN or the denial of access is 2200 based on the client's IP address or other authentication or 2201 authorization-determining mechanisms. 2203 The MAIL command begins a mail transaction. Once started, a mail 2204 transaction consists of a transaction beginning command, one or more 2205 RCPT commands, and a DATA command, in that order. A mail transaction 2206 may be aborted by the RSET, a new EHLO, or the QUIT command. There 2207 may be zero or more transactions in a session. MAIL MUST NOT be sent 2208 if a mail transaction is already open, i.e., it should be sent only 2209 if no mail transaction had been started in the session, or if the 2210 previous one successfully concluded with a successful DATA command, 2211 or if the previous one was aborted, e.g., with a RSET or new EHLO. 2212 [[CREF14: [5321bis] 2821ter note: see comment about changing this 2213 convoluted discussion to talk about 'mail transaction' above. 2214 --Jck]] 2216 If the transaction beginning command argument is not acceptable, a 2217 501 failure reply MUST be returned and the SMTP server MUST stay in 2218 the same state. If the commands in a transaction are out of order to 2219 the degree that they cannot be processed by the server, a 503 failure 2220 reply MUST be returned and the SMTP server MUST stay in the same 2221 state. 2223 The last command in a session MUST be the QUIT command. The QUIT 2224 command SHOULD be used by the client SMTP to request connection 2225 closure, even when no session opening command was sent and accepted. 2227 4.1.5. Private-Use Commands 2229 As specified in Section 2.2.2, commands starting in "X" may be used 2230 by bilateral agreement between the client (sending) and server 2231 (receiving) SMTP agents. An SMTP server that does not recognize such 2232 a command is expected to reply with "500 Command not recognized". An 2233 extended SMTP server MAY list the feature names associated with these 2234 private commands in the response to the EHLO command. 2236 Commands sent or accepted by SMTP systems that do not start with "X" 2237 MUST conform to the requirements of Section 2.2.2. 2239 4.2. SMTP Replies 2241 Replies to SMTP commands serve to ensure the synchronization of 2242 requests and actions in the process of mail transfer and to guarantee 2243 that the SMTP client always knows the state of the SMTP server. 2244 Every command MUST generate exactly one reply. 2246 The details of the command-reply sequence are described in 2247 Section 4.3. 2249 An SMTP reply consists of a three digit number (transmitted as three 2250 numeric characters) followed by some text unless specified otherwise 2251 in this document. The number is for use by automata to determine 2252 what state to enter next; the text is for the human user. The three 2253 digits contain enough encoded information that the SMTP client need 2254 not examine the text and may either discard it or pass it on to the 2255 user, as appropriate. Exceptions are as noted elsewhere in this 2256 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2257 are associated with message text that must be parsed and interpreted 2258 by machines. In the general case, the text may be receiver dependent 2259 and context dependent, so there are likely to be varying texts for 2260 each reply code. A discussion of the theory of reply codes is given 2261 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2262 three-digit code, , one line of text, and , or a multiline 2263 reply (as defined in the same section). Since, in violation of this 2264 specification, the text is sometimes not sent, clients that do not 2265 receive it SHOULD be prepared to process the code alone (with or 2266 without a trailing space character). Only the EHLO, EXPN, and HELP 2267 commands are expected to result in multiline replies in normal 2268 circumstances; however, multiline replies are allowed for any 2269 command. 2271 In ABNF, server responses are: 2273 Greeting = ( "220 " (Domain / address-literal) 2274 [ SP textstring ] CRLF ) / 2275 ( "220-" (Domain / address-literal) 2276 [ SP textstring ] CRLF 2277 *( "220-" [ textstring ] CRLF ) 2278 "220" [ SP textstring ] CRLF ) 2280 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2282 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2283 Reply-code [ SP textstring ] CRLF 2285 Reply-code = %x32-35 %x30-35 %x30-39 2286 where "Greeting" appears only in the 220 response that announces that 2287 the server is opening its part of the connection. (Other possible 2288 server responses upon connection follow the syntax of Reply-line.) 2290 An SMTP server SHOULD send only the reply codes listed in this 2291 document or additions to the list as discussed below. 2292 [[CREF15: [5321bis] 20140804: New text to clear up ambiguity.]] 2293 An SMTP server SHOULD use the text shown in the examples whenever 2294 appropriate. 2296 An SMTP client MUST determine its actions only by the reply code, not 2297 by the text (except for the "change of address" 251 and 551 and, if 2298 necessary, 220, 221, and 421 replies); in the general case, any text, 2299 including no text at all (although senders SHOULD NOT send bare 2300 codes), MUST be acceptable. The space (blank) following the reply 2301 code is considered part of the text. Whenever possible, a sender- 2302 SMTP SHOULD test the first digit (severity indication) of a reply 2303 code it receives. 2304 [[CREF16: [5321bis] 20141209 [[Note in Draft: What is that sentence 2305 supposed to be tell us? Test the first digit and examine the others 2306 only if necessary? Note the interaction between this and various 2307 flaps about adding new codes.]]]] 2309 The list of codes that appears below MUST NOT be construed as 2310 permanent. While the addition of new codes should be a rare and 2311 significant activity, with supplemental information in the textual 2312 part of the response (including enhanced status codes [34] and the 2313 successors to that specification) 2314 [[CREF17: [5321bis] 20140802: New text for clarity]] 2315 being preferred, new codes may be added as the result of new 2316 Standards or Standards-Track specifications. Consequently, a sender- 2317 SMTP MUST be prepared to handle codes not specified in this document 2318 and MUST do so by interpreting the first digit only. 2320 In the absence of extensions negotiated with the client, SMTP servers 2321 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2322 or 5. Clients that receive such out-of-range codes SHOULD normally 2323 treat them as fatal errors and terminate the mail transaction. 2325 4.2.1. Reply Code Severities and Theory 2327 The three digits of the reply each have a special significance. The 2328 first digit denotes whether the response is good, bad, or incomplete. 2329 An unsophisticated SMTP client, or one that receives an unexpected 2330 code, will be able to determine its next action (proceed as planned, 2331 redo, retrench, etc.) by examining this first digit. An SMTP client 2332 that wants to know approximately what kind of error occurred (e.g., 2333 mail system error, command syntax error) may examine the second 2334 digit. The third digit and any supplemental information that may be 2335 present is reserved for the finest gradation of information. 2337 There are four values for the first digit of the reply code: 2339 2yz Positive Completion reply 2340 The requested action has been successfully completed. A new 2341 request may be initiated. 2343 3yz Positive Intermediate reply 2344 The command has been accepted, but the requested action is being 2345 held in abeyance, pending receipt of further information. The 2346 SMTP client should send another command specifying this 2347 information. This reply is used in command sequence groups (i.e., 2348 in DATA). 2350 4yz Transient Negative Completion reply 2351 The command was not accepted, and the requested action did not 2352 occur. However, the error condition is temporary, and the action 2353 may be requested again. The sender should return to the beginning 2354 of the command sequence (if any). It is difficult to assign a 2355 meaning to "transient" when two different sites (receiver- and 2356 sender-SMTP agents) must agree on the interpretation. Each reply 2357 in this category might have a different time value, but the SMTP 2358 client SHOULD try again. A rule of thumb to determine whether a 2359 reply fits into the 4yz or the 5yz category (see below) is that 2360 replies are 4yz if they can be successful if repeated without any 2361 change in command form or in properties of the sender or receiver 2362 (that is, the command is repeated identically and the receiver 2363 does not put up a new implementation). 2365 5yz Permanent Negative Completion reply 2366 The command was not accepted and the requested action did not 2367 occur. The SMTP client SHOULD NOT repeat the exact request (in 2368 the same sequence). Even some "permanent" error conditions can be 2369 corrected, so the human user may want to direct the SMTP client to 2370 reinitiate the command sequence by direct action at some point in 2371 the future (e.g., after the spelling has been changed, or the user 2372 has altered the account status). 2374 It is worth noting that the file transfer protocol (FTP) [14] uses a 2375 very similar code architecture and that the SMTP codes are based on 2376 the FTP model. However, SMTP uses a one-command, one-response model 2377 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2378 SMTP model. 2380 The second digit encodes responses in specific categories: 2382 x0z Syntax: These replies refer to syntax errors, syntactically 2383 correct commands that do not fit any functional category, and 2384 unimplemented or superfluous commands. 2386 x1z Information: These are replies to requests for information, such 2387 as status or help. 2389 x2z Connections: These are replies referring to the transmission 2390 channel. 2392 x3z Unspecified. 2394 x4z Unspecified. 2396 x5z Mail system: These replies indicate the status of the receiver 2397 mail system vis-a-vis the requested transfer or other mail system 2398 action. 2400 The third digit gives a finer gradation of meaning in each category 2401 specified by the second digit. The list of replies illustrates this. 2402 Each reply text is recommended rather than mandatory, and may even 2403 change according to the command with which it is associated. On the 2404 other hand, the reply codes must strictly follow the specifications 2405 in this section. Receiver implementations should not invent new 2406 codes for slightly different situations from the ones described here, 2407 but rather adapt codes already defined. 2409 For example, a command such as NOOP, whose successful execution does 2410 not offer the SMTP client any new information, will return a 250 2411 reply. The reply is 502 when the command requests an unimplemented 2412 non-site-specific action. A refinement of that is the 504 reply for 2413 a command that is implemented, but that requests an unimplemented 2414 parameter. 2416 The reply text may be longer than a single line; in these cases the 2417 complete text must be marked so the SMTP client knows when it can 2418 stop reading the reply. This requires a special format to indicate a 2419 multiple line reply. 2421 The format for multiline replies requires that every line, except the 2422 last, begin with the reply code, followed immediately by a hyphen, 2423 "-" (also known as minus), followed by text. The last line will 2424 begin with the reply code, followed immediately by , optionally 2425 some text, and . As noted above, servers SHOULD send the 2426 if subsequent text is not sent, but clients MUST be prepared for it 2427 to be omitted. 2429 For example: 2431 250-First line 2432 250-Second line 2433 250-234 Text beginning with numbers 2434 250 The last line 2436 In a multiline reply, the reply code on each of the lines MUST be the 2437 same. It is reasonable for the client to rely on this, so it can 2438 make processing decisions based on the code in any line, assuming 2439 that all others will be the same. In a few cases, there is important 2440 data for the client in the reply "text". The client will be able to 2441 identify these cases from the current context. 2443 4.2.2. Reply Codes by Function Groups 2445 500 Syntax error, command unrecognized (This may include errors such 2446 as command line too long) 2448 501 Syntax error in parameters or arguments 2450 502 Command not implemented (see Section 4.2.4.1) 2452 503 Bad sequence of commands 2454 504 Command parameter not implemented 2456 211 System status, or system help reply 2458 214 Help message (Information on how to use the receiver or the 2459 meaning of a particular non-standard command; this reply is useful 2460 only to the human user) 2462 220 Service ready 2464 221 Service closing transmission channel 2466 421 Service not available, closing transmission channel 2467 (This may be a reply to any command if the service knows it must 2468 shut down) 2470 hangText="521"> No mail service here. [[CREF18: 2471 [5321bis]20140804: Specific code introduced with RFC 1846, updated 2472 and specified in draft-klensin-smtp-521code.]] 2474 556 No mail service at this domain. [[CREF19: [5321bis] 20140912: 2475 Specific code introduced in draft-klensin-smtp-521code-02 (RFC 2476 7504), largely for nullMX]] 2478 250 Requested mail action okay, completed 2480 251 User not local; will forward to (See Section 3.4) 2482 252 Cannot VRFY user, but will accept message and attempt delivery 2483 (See Section 3.5.3) 2485 455 Server unable to accommodate parameters 2487 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2489 450 Requested mail action not taken: mailbox unavailable (e.g., 2490 mailbox busy or temporarily blocked for policy reasons) 2492 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2493 not found, no access, or command rejected for policy reasons) 2495 451 Requested action aborted: error in processing 2497 551 User not local; please try (See Section 3.4) 2499 452 Requested action not taken: insufficient system storage 2501 552 Requested mail action aborted: exceeded storage allocation 2503 553 Requested action not taken: mailbox name not allowed (e.g., 2504 mailbox syntax incorrect) 2506 354 Start mail input; end with . 2508 554 Transaction failed (Or, in the case of a connection-opening 2509 response, "No SMTP service here") 2510 [[CREF20: [5321bis] [[Note in Draft: Revise above statement in the 2511 light of new 521 code??]] ]] 2513 4.2.3. Reply Codes in Numeric Order 2515 211 System status, or system help reply 2517 214 Help message (Information on how to use the receiver or the 2518 meaning of a particular non-standard command; this reply is useful 2519 only to the human user) 2521 220 Service ready 2523 221 Service closing transmission channel 2525 250 Requested mail action okay, completed 2526 251 User not local; will forward to (See Section 3.4) 2528 252 Cannot VRFY user, but will accept message and attempt delivery 2529 (See Section 3.5.3) 2531 354 Start mail input; end with . 2533 421 Service not available, closing transmission channel 2534 (This may be a reply to any command if the service knows it must 2535 shut down) 2537 450 Requested mail action not taken: mailbox unavailable (e.g., 2538 mailbox busy or temporarily blocked for policy reasons) 2540 451 Requested action aborted: local error in processing 2542 452 Requested action not taken: insufficient system storage 2544 455 Server unable to accommodate parameters 2546 500 Syntax error, command unrecognized (This may include errors such 2547 as command line too long) 2549 501 Syntax error in parameters or arguments 2551 502 Command not implemented (see Section 4.2.4.1) 2553 503 Bad sequence of commands 2555 504 Command parameter not implemented 2557 521 No mail service 2559 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2560 not found, no access, or command rejected for policy reasons) 2562 551 User not local; please try (See Section 3.4) 2564 552 Requested mail action aborted: exceeded storage allocation 2566 553 Requested action not taken: mailbox name not allowed (e.g., 2567 mailbox syntax incorrect) 2569 554 Transaction failed (Or, in the case of a connection-opening 2570 response, "No SMTP service here") 2572 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2574 556 No mail service at this domain. 2576 4.2.4. Some specific code situations and relationships 2578 4.2.4.1. Reply Code 502 2580 Questions have been raised as to when reply code 502 (Command not 2581 implemented) SHOULD be returned in preference to other codes. 502 2582 SHOULD be used when the command is actually recognized by the SMTP 2583 server, but not implemented. If the command is not recognized, code 2584 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2585 capabilities in response to EHLO for which they will return 502 (or 2586 500) replies. 2588 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2590 [[CREF21: [5321bis] This section is new with 5321bis. ]] 2592 Codes 521, 554, and 556 are all used to report different types of "no 2593 mail accepted" situations. They differ as follows. 521 is an 2594 indication from a system answering on the SMTP port that it does not 2595 support SMTP service (a so-called "dummy server" as discussed in RFC 2596 1846 [19] and elsewhere). Obviously, it requires that system exist 2597 and that a connection can be made successfully to it. Because a 2598 system that does not accept any mail cannot meaningfully accept a 2599 RCPT command, any commands (other than QUIT) issued after an SMTP 2600 server has issued a 521 reply are client (sender) errors. 556 is 2601 used by a message submission or intermediate SMTP system (see 2602 Section 1.1) to report that it cannot forward the message further 2603 because it knows (e.g., from a DNS entry [51]) that the recipient 2604 domain does not accept mail. It would normally be used in response 2605 to a RCPT or similar (extension) command when the SMTP system 2606 identifies a domain that it can (or has) determined never accepts 2607 mail. Other codes, including 554 and the temporary 450, are used for 2608 more transient situations and situations in which an SMTP server 2609 cannot or will not deliver to (or accept mail for) a particular 2610 system or mailbox for policy reasons rather than ones directly 2611 related to SMTP processing. 2613 4.2.4.3. Reply Codes after DATA and the Subsequent . 2615 When an SMTP server returns a positive completion status (2yz code) 2616 after the DATA command is completed with ., it accepts 2617 responsibility for: 2619 o delivering the message (if the recipient mailbox exists), or 2621 o if attempts to deliver the message fail due to transient 2622 conditions, retrying delivery some reasonable number of times at 2623 intervals as specified in Section 4.5.4. 2625 o if attempts to deliver the message fail due to permanent 2626 conditions, or if repeated attempts to deliver the message fail 2627 due to transient conditions, returning appropriate notification to 2628 the sender of the original message (using the address in the SMTP 2629 MAIL command). 2631 When an SMTP server returns a temporary error status (4yz) code after 2632 the DATA command is completed with ., it MUST NOT make a 2633 subsequent attempt to deliver that message. The SMTP client retains 2634 responsibility for the delivery of that message and may either return 2635 it to the user or requeue it for a subsequent attempt (see 2636 Section 4.5.4.1). 2638 The user who originated the message SHOULD be able to interpret the 2639 return of a transient failure status (by mail message or otherwise) 2640 as a non-delivery indication, just as a permanent failure would be 2641 interpreted. If the client SMTP successfully handles these 2642 conditions, the user will not receive such a reply. 2644 When an SMTP server returns a permanent error status (5yz) code after 2645 the DATA command is completed with ., it MUST NOT make 2646 any subsequent attempt to deliver the message. As with temporary 2647 error status codes, the SMTP client retains responsibility for the 2648 message, but SHOULD NOT again attempt delivery to the same server 2649 without user review of the message and response and appropriate 2650 intervention. 2652 4.3. Sequencing of Commands and Replies 2654 4.3.1. Sequencing Overview 2656 The communication between the sender and receiver is an alternating 2657 dialogue, controlled by the sender. As such, the sender issues a 2658 command and the receiver responds with a reply. Unless other 2659 arrangements are negotiated through service extensions, the sender 2660 MUST wait for this response before sending further commands. One 2661 important reply is the connection greeting. Normally, a receiver 2662 will send a 220 "Service ready" reply when the connection is 2663 completed. The sender SHOULD wait for this greeting message before 2664 sending any commands. 2666 Note: all the greeting-type replies have the official name (the 2667 fully-qualified primary domain name) of the server host as the first 2668 word following the reply code. Sometimes the host will have no 2669 meaningful name. See Section 4.1.3 for a discussion of alternatives 2670 in these situations. 2672 For example, 2674 220 ISIF.USC.EDU Service ready 2676 or 2678 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2680 or 2682 220 [10.0.0.1] Clueless host service ready 2684 The table below lists alternative success and failure replies for 2685 each command. These SHOULD be strictly adhered to. A receiver MAY 2686 substitute text in the replies, but the meanings and actions implied 2687 by the code numbers and by the specific command reply sequence MUST 2688 be preserved. However, in order to provide robustness as SMTP is 2689 extended and evolves, the discussion in Section 4.2.1 still applies: 2690 all SMTP clients MUST be prepared to accept any code that conforms to 2691 the discussion in that section and MUST be prepared to interpret it 2692 on the basis of its first digit only. [[CREF22: [5321bis] 20140914: 2693 Above sentence is new text based on yet another round of discussions 2694 about "invalid codes".]] 2696 4.3.2. Command-Reply Sequences 2698 Each command is listed with its usual possible replies. The prefixes 2699 used before the possible replies are "I" for intermediate, "S" for 2700 success, and "E" for error. Since some servers may generate other 2701 replies under special circumstances, and to allow for future 2702 extension, SMTP clients SHOULD, when possible, interpret only the 2703 first digit of the reply and MUST be prepared to deal with 2704 unrecognized reply codes by interpreting the first digit only. 2705 Unless extended using the mechanisms described in Section 2.2, SMTP 2706 servers MUST NOT transmit reply codes to an SMTP client that are 2707 other than three digits or that do not start in a digit between 2 and 2708 5 inclusive. 2710 These sequencing rules and, in principle, the codes themselves, can 2711 be extended or modified by SMTP extensions offered by the server and 2712 accepted (requested) by the client. However, if the target is more 2713 precise granularity in the codes, rather than codes for completely 2714 new purposes, the system described in RFC 3463 [34] SHOULD be used in 2715 preference to the invention of new codes. 2717 In addition to the codes listed below, any SMTP command can return 2718 any of the following codes if the corresponding unusual circumstances 2719 are encountered: 2721 500 For the "command line too long" case or if the command name was 2722 not recognized. Note that producing a "command not recognized" 2723 error in response to the required subset of these commands is a 2724 violation of this specification. Similarly, producing a "command 2725 too long" message for a command line shorter than 512 characters 2726 would violate the provisions of Section 4.5.3.1.4. 2728 501 Syntax error in command or arguments. In order to provide for 2729 future extensions, commands that are specified in this document as 2730 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2731 message if arguments are supplied in the absence of EHLO- 2732 advertised extensions. 2734 421 Service shutting down and closing transmission channel 2736 Specific sequences are: 2738 CONNECTION ESTABLISHMENT 2740 S: 220 2741 E: 521, 554 2743 EHLO or HELO 2745 S: 250 2746 E: 504 (a conforming implementation could return this code only 2747 in fairly obscure cases), 550, 502 (permitted only with an old- 2748 style server that does not support EHLO) 2750 MAIL 2752 S: 250 2753 E: 552, 451, 452, 550, 553, 503, 455, 555 2755 RCPT 2757 S: 250, 251 (but see Section 3.4 for discussion of 251 and 551) 2758 E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555 2760 DATA 2762 I: 354 -> data -> S: 250 2764 E: 552, 554, 451, 452 2766 E: 450, 550 (rejections for policy reasons) 2768 E: 503, 554 2770 RSET 2772 S: 250 2774 VRFY 2776 S: 250, 251, 252 2777 E: 550, 551, 553, 502, 504 2779 EXPN 2781 S: 250, 252 2782 E: 550, 500, 502, 504 2784 HELP 2786 S: 211, 214 2787 E: 502, 504 2789 NOOP 2791 S: 250 2793 QUIT 2795 S: 221 2797 4.4. Trace Information 2799 When an SMTP server receives a message for delivery or further 2800 processing, it MUST insert trace (often referred to as "time stamp" 2801 or "Received" information) [[CREF23: [5321bis] See note on 2802 rfc5321bis-00c above]] information at the beginning of the message 2803 content, as discussed in Section 4.1.1.4. 2805 This line MUST be structured as follows: 2807 o The FROM clause, which MUST be supplied in an SMTP environment, 2808 SHOULD contain both (1) the name of the source host as presented 2809 in the EHLO command and (2) an address literal containing the IP 2810 address of the source, determined from the TCP connection. 2812 o The ID clause MAY contain an "@" as suggested in RFC 822, but this 2813 is not required. 2815 o If the FOR clause appears, it MUST contain exactly one 2816 entry, even when multiple RCPT commands have been given. Multiple 2817 s raise some security issues and have been deprecated, see 2818 Section 7.2. 2820 An Internet mail program MUST NOT change or delete a Received: line 2821 that was previously added to the message header section. SMTP 2822 servers MUST prepend Received lines to messages; they MUST NOT change 2823 the order of existing lines or insert Received lines in any other 2824 location. 2826 As the Internet grows, comparability of Received header fields is 2827 important for detecting problems, especially slow relays. SMTP 2828 servers that create Received header fields SHOULD use explicit 2829 offsets in the dates (e.g., -0800), rather than time zone names of 2830 any type. Local time (with an offset) SHOULD be used rather than UT 2831 when feasible. This formulation allows slightly more information 2832 about local circumstances to be specified. If UT is needed, the 2833 receiver need merely do some simple arithmetic to convert the values. 2834 Use of UT loses information about the time zone-location of the 2835 server. If it is desired to supply a time zone name, it SHOULD be 2836 included in a comment. 2838 When the delivery SMTP server makes the "final delivery" of a 2839 message, it inserts a return-path line at the beginning of the mail 2840 data. This use of return-path is required; mail systems MUST support 2841 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2843 has left the SMTP environment. Normally, this would mean it had been 2844 delivered to the destination user or an associated mail drop, but in 2845 some cases it may be further processed and transmitted by another 2846 mail system. 2848 It is possible for the mailbox in the return path to be different 2849 from the actual sender's mailbox, for example, if error responses are 2850 to be delivered to a special error handling mailbox rather than to 2851 the message sender. When mailing lists are involved, this 2852 arrangement is common and useful as a means of directing errors to 2853 the list maintainer rather than the message originator. 2855 The text above implies that the final mail data will begin with a 2856 return path line, followed by one or more time stamp lines. These 2857 lines will be followed by the rest of the mail data: first the 2858 balance of the mail header section and then the body (RFC 5322 [11]). 2860 It is sometimes difficult for an SMTP server to determine whether or 2861 not it is making final delivery since forwarding or other operations 2862 may occur after the message is accepted for delivery. Consequently, 2863 any further (forwarding, gateway, or relay) systems MAY remove the 2864 return path and rebuild the MAIL command as needed to ensure that 2865 exactly one such line appears in a delivered message. 2867 A message-originating SMTP system SHOULD NOT send a message that 2868 already contains a Return-path header field. SMTP servers performing 2869 a relay function MUST NOT inspect the message data, and especially 2870 not to the extent needed to determine if Return-path header fields 2871 are present. SMTP servers making final delivery MAY remove Return- 2872 path header fields before adding their own. 2874 The primary purpose of the Return-path is to designate the address to 2875 which messages indicating non-delivery or other mail system failures 2876 are to be sent. For this to be unambiguous, exactly one return path 2877 SHOULD be present when the message is delivered. Systems using RFC 2878 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2879 address, associated with the transport envelope, to which error 2880 reports (e.g., non-delivery messages) should be sent. 2882 Historical note: Text in RFC 822 that appears to contradict the use 2883 of the Return-path header field (or the envelope reverse-path address 2884 from the MAIL command) as the destination for error messages is not 2885 applicable on the Internet. The reverse-path address (as copied into 2886 the Return-path) MUST be used as the target of any mail containing 2887 delivery error messages. 2889 In particular: 2891 o a gateway from SMTP -> elsewhere SHOULD insert a return-path 2892 header field, unless it is known that the "elsewhere" transport 2893 also uses Internet domain addresses and maintains the envelope 2894 sender address separately. 2896 o a gateway from elsewhere -> SMTP SHOULD delete any return-path 2897 header field present in the message, and either copy that 2898 information to the SMTP envelope or combine it with information 2899 present in the envelope of the other transport system to construct 2900 the reverse-path argument to the MAIL command in the SMTP 2901 envelope. 2903 The server must give special treatment to cases in which the 2904 processing following the end of mail data indication is only 2905 partially successful. This could happen if, after accepting several 2906 recipients and the mail data, the SMTP server finds that the mail 2907 data could be successfully delivered to some, but not all, of the 2908 recipients. In such cases, the response to the DATA command MUST be 2909 an OK reply. However, the SMTP server MUST compose and send an 2910 "undeliverable mail" notification message to the originator of the 2911 message. 2913 A single notification listing all of the failed recipients or 2914 separate notification messages MUST be sent for each failed 2915 recipient. For economy of processing by the sender, the former 2916 SHOULD be used when possible. Note that the key difference between 2917 handling aliases (Section 3.9.1) and forwarding (this subsection) is 2918 the change to the backward-pointing address in this case. All 2919 notification messages about undeliverable mail MUST be sent using the 2920 MAIL command and MUST use a null return path as discussed in 2921 Section 3.6. 2923 The time stamp line and the return path line are formally defined as 2924 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2925 [11]): 2927 Return-path-line = "Return-Path:" FWS Reverse-path 2929 Time-stamp-line = "Received:" FWS Stamp 2931 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2932 FWS date-time 2933 ; where "date-time" is as defined in RFC 5322 [11] 2934 ; but the "obs-" forms, especially two-digit 2935 ; years, are prohibited in SMTP and MUST NOT be used. 2937 From-domain = "FROM" FWS Extended-Domain 2939 By-domain = CFWS "BY" FWS Extended-Domain 2941 Extended-Domain = Domain / 2942 ( Domain FWS "(" TCP-info ")" ) / 2943 ( address-literal FWS "(" TCP-info ")" ) 2945 TCP-info = address-literal / ( Domain FWS address-literal ) 2946 ; Information derived by server from TCP connection 2947 ; not client EHLO. 2949 Opt-info = [Via] [With] [ID] [For] 2950 [Additional-Registered-Clauses] 2952 Via = CFWS "VIA" FWS Link 2954 With = CFWS "WITH" FWS Protocol 2956 ID = CFWS "ID" FWS ( Atom / msg-id ) 2957 ; msg-id is defined in RFC 5322 [11] 2959 For = CFWS "FOR" FWS ( Path / Mailbox ) 2961 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 2962 [[CREF24: [5321bis] 5321 errata #1683, 20090215, 2963 Roberto Javier Godoy, rjgodoy@fich.unl.edu.ar]] 2964 ; Additional standard clauses may be added in this 2965 ; location by future standards and registration with 2966 ; IANA. SMTP servers SHOULD NOT use unregistered 2967 ; names. See Section 8. 2969 Link = "TCP" / Addtl-Link 2971 Addtl-Link = Atom 2972 ; Additional standard names for links are 2973 ; registered with the Internet Assigned Numbers 2974 ; Authority (IANA). "Via" is primarily of value 2975 ; with non-Internet transports. SMTP servers 2976 ; SHOULD NOT use unregistered names. 2978 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 2980 Addtl-Protocol = Atom 2981 ; Additional standard names for protocols are 2982 ; registered with the Internet Assigned Numbers 2983 ; Authority (IANA) in the "mail parameters" 2984 ; registry [7]. SMTP servers SHOULD NOT 2985 ; use unregistered names. 2987 4.5. Additional Implementation Issues 2989 4.5.1. Minimum Implementation 2991 In order to make SMTP workable, the following minimum implementation 2992 MUST be provided by all receivers. The following commands MUST be 2993 supported to conform to this specification: 2995 EHLO 2996 HELO 2997 MAIL 2998 RCPT 2999 DATA 3000 RSET 3001 NOOP 3002 QUIT 3003 VRFY 3005 Any system that includes an SMTP server supporting mail relaying or 3006 delivery MUST support the reserved mailbox "postmaster" as a case- 3007 insensitive local name. This postmaster address is not strictly 3008 necessary if the server always returns 554 on connection opening (as 3009 described in Section 3.1). The requirement to accept mail for 3010 postmaster implies that RCPT commands that specify a mailbox for 3011 postmaster at any of the domains for which the SMTP server provides 3012 mail service, as well as the special case of "RCPT TO:" 3013 (with no domain specification), MUST be supported. 3015 SMTP systems are expected to make every reasonable effort to accept 3016 mail directed to Postmaster from any other system on the Internet. 3017 In extreme cases -- such as to contain a denial of service attack or 3018 other breach of security -- an SMTP server may block mail directed to 3019 Postmaster. However, such arrangements SHOULD be narrowly tailored 3020 so as to avoid blocking messages that are not part of such attacks. 3022 4.5.2. Transparency 3024 Without some provision for data transparency, the character sequence 3025 "." ends the mail text and cannot be sent by the user. 3026 In general, users are not aware of such "forbidden" sequences. To 3027 allow all user composed text to be transmitted transparently, the 3028 following procedures are used: 3030 o Before sending a line of mail text, the SMTP client checks the 3031 first character of the line. If it is a period, one additional 3032 period is inserted at the beginning of the line. 3034 o When a line of mail text is received by the SMTP server, it checks 3035 the line. If the line is composed of a single period, it is 3036 treated as the end of mail indicator. If the first character is a 3037 period and there are other characters on the line, the first 3038 character is deleted. 3040 The mail data may contain any of the 128 ASCII characters. All 3041 characters are to be delivered to the recipient's mailbox, including 3042 spaces, vertical and horizontal tabs, and other control characters. 3043 If the transmission channel provides an 8-bit byte (octet) data 3044 stream, the 7-bit ASCII codes are transmitted, right justified, in 3045 the octets, with the high-order bits cleared to zero. See 3046 Section 3.6 for special treatment of these conditions in SMTP systems 3047 serving a relay function. 3049 In some systems, it may be necessary to transform the data as it is 3050 received and stored. This may be necessary for hosts that use a 3051 different character set than ASCII as their local character set, that 3052 store data in records rather than strings, or which use special 3053 character sequences as delimiters inside mailboxes. If such 3054 transformations are necessary, they MUST be reversible, especially if 3055 they are applied to mail being relayed. 3057 4.5.3. Sizes and Timeouts 3059 4.5.3.1. Size Limits and Minimums 3061 There are several objects that have required minimum/maximum sizes. 3062 Every implementation MUST be able to receive objects of at least 3063 these sizes. Objects larger than these sizes SHOULD be avoided when 3064 possible. However, some Internet mail constructs such as encoded 3065 X.400 addresses (RFC 2156 [26]) will often require larger objects. 3066 Clients MAY attempt to transmit these, but MUST be prepared for a 3067 server to reject them if they cannot be handled by it. To the 3068 maximum extent possible, implementation techniques that impose no 3069 limits on the length of these objects should be used. 3071 Extensions to SMTP may involve the use of characters that occupy more 3072 than a single octet each. This section therefore specifies lengths 3073 in octets where absolute lengths, rather than character counts, are 3074 intended. 3076 [[CREF25: [5321bis] [[Note in Draft: Klensin 20191126: Given the 3077 controversy on the SMTP mailing list between 20191123 and now about 3078 maximum lengths, is the above adequate or is further tuning of the 3079 limit text below needed? ]]]] 3081 4.5.3.1.1. Local-part 3083 The maximum total length of a user name or other local-part is 64 3084 octets. 3086 4.5.3.1.2. Domain 3088 The maximum total length of a domain name or number is 255 octets. 3090 4.5.3.1.3. Path 3092 The maximum total length of a reverse-path or forward-path is 256 3093 octets (including the punctuation and element separators). 3095 4.5.3.1.4. Command Line 3097 The maximum total length of a command line including the command word 3098 and the is 512 octets. SMTP extensions may be used to 3099 increase this limit. 3101 4.5.3.1.5. Reply Line 3103 The maximum total length of a reply line including the reply code and 3104 the is 512 octets. More information may be conveyed through 3105 multiple-line replies. 3107 4.5.3.1.6. Text Line 3109 The maximum total length of a text line including the is 1000 3110 octets (not counting the leading dot duplicated for transparency). 3111 This number may be increased by the use of SMTP Service Extensions. 3113 4.5.3.1.7. Message Content 3115 The maximum total length of a message content (including any message 3116 header section as well as the message body) MUST BE at least 64K 3117 octets. Since the introduction of Internet Standards for multimedia 3118 mail (RFC 2045 [24]), message lengths on the Internet have grown 3119 dramatically, and message size restrictions should be avoided if at 3120 all possible. SMTP server systems that must impose restrictions 3121 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3122 SMTP client systems that will send large messages SHOULD utilize it 3123 when possible. 3125 4.5.3.1.8. Recipient Buffer 3127 The minimum total number of recipients that MUST be buffered is 100 3128 recipients. Rejection of messages (for excessive recipients) with 3129 fewer than 100 RCPT commands is a violation of this specification. 3130 The general principle that relaying SMTP server MUST NOT, and 3131 delivery SMTP servers SHOULD NOT, perform validation tests on message 3132 header fields suggests that messages SHOULD NOT be rejected based on 3133 the total number of recipients shown in header fields. A server that 3134 imposes a limit on the number of recipients MUST behave in an orderly 3135 fashion, such as rejecting additional addresses over its limit rather 3136 than silently discarding addresses previously accepted. A client 3137 that needs to deliver a message containing over 100 RCPT commands 3138 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3139 server declines to accept more than 100 recipients in a single 3140 message. 3142 4.5.3.1.9. Treatment When Limits Exceeded 3144 Errors due to exceeding these limits may be reported by using the 3145 reply codes. Some examples of reply codes are: 3147 500 Line too long. 3149 or 3151 501 Path too long 3153 or 3155 452 Too many recipients (see below) 3157 or 3159 552 Too much mail data. 3161 4.5.3.1.10. Too Many Recipients Code 3163 RFC 821 [3] incorrectly listed the error where an SMTP server 3164 exhausts its implementation limit on the number of RCPT commands 3165 ("too many recipients") as having reply code 552. The correct reply 3166 code for this condition is 452. Clients SHOULD treat a 552 code in 3167 this case as a temporary, rather than permanent, failure so the logic 3168 below works. 3170 When a conforming SMTP server encounters this condition, it has at 3171 least 100 successful RCPT commands in its recipient buffer. If the 3172 server is able to accept the message, then at least these 100 3173 addresses will be removed from the SMTP client's queue. When the 3174 client attempts retransmission of those addresses that received 452 3175 responses, at least 100 of these will be able to fit in the SMTP 3176 server's recipient buffer. Each retransmission attempt that is able 3177 to deliver anything will be able to dispose of at least 100 of these 3178 recipients. 3180 If an SMTP server has an implementation limit on the number of RCPT 3181 commands and this limit is exhausted, it MUST use a response code of 3182 452 (but the client SHOULD also be prepared for a 552, as noted 3183 above). If the server has a configured site-policy limitation on the 3184 number of RCPT commands, it MAY instead use a 5yz response code. In 3185 particular, if the intent is to prohibit messages with more than a 3186 site-specified number of recipients, rather than merely limit the 3187 number of recipients in a given mail transaction, it would be 3188 reasonable to return a 503 response to any DATA command received 3189 subsequent to the 452 (or 552) code or to simply return the 503 after 3190 DATA without returning any previous negative response. 3192 4.5.3.2. Timeouts 3194 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3195 command timeouts rather than somehow trying to time the entire mail 3196 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3197 without recompiling the SMTP code. To implement this, a timer is set 3198 for each SMTP command and for each buffer of the data transfer. The 3199 latter means that the overall timeout is inherently proportional to 3200 the size of the message. 3202 Based on extensive experience with busy mail-relay hosts, the minimum 3203 per-command timeout values SHOULD be as follows: 3205 4.5.3.2.1. Initial 220 Message: 5 Minutes 3207 An SMTP client process needs to distinguish between a failed TCP 3208 connection and a delay in receiving the initial 220 greeting message. 3209 Many SMTP servers accept a TCP connection but delay delivery of the 3210 220 message until their system load permits more mail to be 3211 processed. 3213 4.5.3.2.2. MAIL Command: 5 Minutes 3215 4.5.3.2.3. RCPT Command: 5 Minutes 3217 A longer timeout is required if processing of mailing lists and 3218 aliases is not deferred until after the message was accepted. 3220 4.5.3.2.4. DATA Initiation: 2 Minutes 3222 This is while awaiting the "354 Start Input" reply to a DATA command. 3224 4.5.3.2.5. Data Block: 3 Minutes 3226 This is while awaiting the completion of each TCP SEND call 3227 transmitting a chunk of data. 3229 4.5.3.2.6. DATA Termination: 10 Minutes. 3231 This is while awaiting the "250 OK" reply. When the receiver gets 3232 the final period terminating the message data, it typically performs 3233 processing to deliver the message to a user mailbox. A spurious 3234 timeout at this point would be very wasteful and would typically 3235 result in delivery of multiple copies of the message, since it has 3236 been successfully sent and the server has accepted responsibility for 3237 delivery. See Section 6.1 for additional discussion. 3239 4.5.3.2.7. Server Timeout: 5 Minutes. 3241 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3242 is awaiting the next command from the sender. 3244 4.5.4. Retry Strategies 3246 The common structure of a host SMTP implementation includes user 3247 mailboxes, one or more areas for queuing messages in transit, and one 3248 or more daemon processes for sending and receiving mail. The exact 3249 structure will vary depending on the needs of the users on the host 3250 and the number and size of mailing lists supported by the host. We 3251 describe several optimizations that have proved helpful, particularly 3252 for mailers supporting high traffic levels. 3254 Any queuing strategy MUST include timeouts on all activities on a 3255 per-command basis. A queuing strategy MUST NOT send error messages 3256 in response to error messages under any circumstances. 3258 4.5.4.1. Sending Strategy 3260 The general model for an SMTP client is one or more processes that 3261 periodically attempt to transmit outgoing mail. In a typical system, 3262 the program that composes a message has some method for requesting 3263 immediate attention for a new piece of outgoing mail, while mail that 3264 cannot be transmitted immediately MUST be queued and periodically 3265 retried by the sender. A mail queue entry will include not only the 3266 message itself but also the envelope information. 3268 The sender MUST delay retrying a particular destination after one 3269 attempt has failed. In general, the retry interval SHOULD be at 3270 least 30 minutes; however, more sophisticated and variable strategies 3271 will be beneficial when the SMTP client can determine the reason for 3272 non-delivery. 3274 Retries continue until the message is transmitted or the sender gives 3275 up; the give-up time generally needs to be at least 4-5 days. It MAY 3276 be appropriate to set a shorter maximum number of retries for non- 3277 delivery notifications and equivalent error messages than for 3278 standard messages. The parameters to the retry algorithm MUST be 3279 configurable. 3281 A client SHOULD keep a list of hosts it cannot reach and 3282 corresponding connection timeouts, rather than just retrying queued 3283 mail items. 3285 Experience suggests that failures are typically transient (the target 3286 system or its connection has crashed), favoring a policy of two 3287 connection attempts in the first hour the message is in the queue, 3288 and then backing off to one every two or three hours. 3290 The SMTP client can shorten the queuing delay in cooperation with the 3291 SMTP server. For example, if mail is received from a particular 3292 address, it is likely that mail queued for that host can now be sent. 3293 Application of this principle may, in many cases, eliminate the 3294 requirement for an explicit "send queues now" function such as ETRN, 3295 RFC 1985 [23]. 3297 The strategy may be further modified as a result of multiple 3298 addresses per host (see below) to optimize delivery time versus 3299 resource usage. 3301 An SMTP client may have a large queue of messages for each 3302 unavailable destination host. If all of these messages were retried 3303 in every retry cycle, there would be excessive Internet overhead and 3304 the sending system would be blocked for a long period. Note that an 3305 SMTP client can generally determine that a delivery attempt has 3306 failed only after a timeout of several minutes, and even a one-minute 3307 timeout per connection will result in a very large delay if retries 3308 are repeated for dozens, or even hundreds, of queued messages to the 3309 same host. 3311 At the same time, SMTP clients SHOULD use great care in caching 3312 negative responses from servers. In an extreme case, if EHLO is 3313 issued multiple times during the same SMTP connection, different 3314 answers may be returned by the server. More significantly, 5yz 3315 responses to the MAIL command MUST NOT be cached. 3317 When a mail message is to be delivered to multiple recipients, and 3318 the SMTP server to which a copy of the message is to be sent is the 3319 same for multiple recipients, then only one copy of the message 3320 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3321 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3322 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3323 are very many addresses, a limit on the number of RCPT commands per 3324 MAIL command MAY be imposed. This efficiency feature SHOULD be 3325 implemented. 3327 Similarly, to achieve timely delivery, the SMTP client MAY support 3328 multiple concurrent outgoing mail transactions. However, some limit 3329 may be appropriate to protect the host from devoting all its 3330 resources to mail. 3332 4.5.4.2. Receiving Strategy 3334 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3335 port (specified by IANA as port 25) at all times. This requires the 3336 support of multiple incoming TCP connections for SMTP. Some limit 3337 MAY be imposed, but servers that cannot handle more than one SMTP 3338 transaction at a time are not in conformance with the intent of this 3339 specification. 3341 As discussed above, when the SMTP server receives mail from a 3342 particular host address, it could activate its own SMTP queuing 3343 mechanisms to retry any mail pending for that host address. 3345 4.5.5. Messages with a Null Reverse-Path 3347 There are several types of notification messages that are required by 3348 existing and proposed Standards to be sent with a null reverse-path, 3349 namely non-delivery notifications as discussed in Section 3.7, other 3350 kinds of Delivery Status Notifications (DSNs, RFC 3461 [33]), and 3351 Message Disposition Notifications (MDNs, RFC 3798 [37]). All of 3352 these kinds of messages are notifications about a previous message, 3353 and they are sent to the reverse-path of the previous mail message. 3354 (If the delivery of such a notification message fails, that usually 3355 indicates a problem with the mail system of the host to which the 3356 notification message is addressed. For this reason, at some hosts 3357 the MTA is set up to forward such failed notification messages to 3358 someone who is able to fix problems with the mail system, e.g., via 3359 the postmaster alias.) 3361 All other types of messages (i.e., any message which is not required 3362 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3363 with a valid, non-null reverse-path. 3365 Implementers of automated email processors should be careful to make 3366 sure that the various kinds of messages with a null reverse-path are 3367 handled correctly. In particular, such systems SHOULD NOT reply to 3368 messages with a null reverse-path, and they SHOULD NOT add a non-null 3369 reverse-path, or change a null reverse-path to a non-null one, to 3370 such messages when forwarding. 3372 5. Address Resolution and Mail Handling 3373 5.1. Locating the Target Host 3375 Once an SMTP client lexically identifies a domain to which mail will 3376 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3377 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3378 [4]). The names are expected to be fully-qualified domain names 3379 (FQDNs): mechanisms for inferring FQDNs from partial names or local 3380 aliases are outside of this specification. Due to a history of 3381 problems, SMTP servers used for initial submission of messages SHOULD 3382 NOT make such inferences (Message Submission Servers [42] have 3383 somewhat more flexibility) and intermediate (relay) SMTP servers MUST 3384 NOT make them. 3386 The lookup first attempts to locate an MX record associated with the 3387 name. If a CNAME record is found, the resulting name is processed as 3388 if it were the initial name. If a non-existent domain error is 3389 returned, this situation MUST be reported as an error. If a 3390 temporary error is returned, the message MUST be queued and retried 3391 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3392 the address is treated as if it was associated with an implicit MX 3393 RR, with a preference of 0, pointing to that host. If MX records are 3394 present, but none of them are usable, or the implicit MX is unusable, 3395 this situation MUST be reported as an error. 3397 If one or more MX RRs are found for a given name, SMTP systems MUST 3398 NOT utilize any address RRs associated with that name unless they are 3399 located using the MX RRs; the "implicit MX" rule above applies only 3400 if there are no MX records present. If MX records are present, but 3401 none of them are usable, this situation MUST be reported as an error. 3403 When a domain name associated with an MX RR is looked up and the 3404 associated data field obtained, the data field of that response MUST 3405 contain a domain name that conforms to the specifications of 3406 Section 2.3.5. 3407 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3408 unfolds, some additional text may be in order here (20140718)]] 3409 That domain name, when queried, MUST return at least one address 3410 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3411 server to which the message should be directed. Any other response, 3412 specifically including a value that will return a CNAME record when 3413 queried, lies outside the scope of this Standard. The prohibition on 3414 labels in the data that resolve to CNAMEs is discussed in more detail 3415 in RFC 2181, Section 10.3 [27]. 3417 When the lookup succeeds, the mapping can result in a list of 3418 alternative delivery addresses rather than a single address, because 3419 of multiple MX records, multihoming, or both. To provide reliable 3420 mail transmission, the SMTP client MUST be able to try (and retry) 3421 each of the relevant addresses in this list in order, until a 3422 delivery attempt succeeds. However, there MAY also be a configurable 3423 limit on the number of alternate addresses that can be tried. In any 3424 case, the SMTP client SHOULD try at least two addresses. 3426 Two types of information are used to rank the host addresses: 3427 multiple MX records, and multihomed hosts. 3429 MX records contain a preference indication that MUST be used in 3430 sorting if more than one such record appears (see below). Lower 3431 numbers are more preferred than higher ones. If there are multiple 3432 destinations with the same preference and there is no clear reason to 3433 favor one (e.g., by recognition of an easily reached address), then 3434 the sender-SMTP MUST randomize them to spread the load across 3435 multiple mail exchangers for a specific organization. 3437 The destination host (perhaps taken from the preferred MX record) may 3438 be multihomed, in which case the domain name resolver will return a 3439 list of alternative IP addresses. It is the responsibility of the 3440 domain name resolver interface to have ordered this list by 3441 decreasing preference if necessary, and the SMTP sender MUST try them 3442 in the order presented. 3444 Although the capability to try multiple alternative addresses is 3445 required, specific installations may want to limit or disable the use 3446 of alternative addresses. The question of whether a sender should 3447 attempt retries using the different addresses of a multihomed host 3448 has been controversial. The main argument for using the multiple 3449 addresses is that it maximizes the probability of timely delivery, 3450 and indeed sometimes the probability of any delivery; the counter- 3451 argument is that it may result in unnecessary resource use. Note 3452 that resource use is also strongly determined by the sending strategy 3453 discussed in Section 4.5.4.1. 3455 If an SMTP server receives a message with a destination for which it 3456 is a designated Mail eXchanger, it MAY relay the message (potentially 3457 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3458 final delivery of the message, or hand it off using some mechanism 3459 outside the SMTP-provided transport environment. Of course, neither 3460 of the latter require that the list of MX records be examined 3461 further. 3463 If it determines that it should relay the message without rewriting 3464 the address, it MUST sort the MX records to determine candidates for 3465 delivery. The records are first ordered by preference, with the 3466 lowest-numbered records being most preferred. The relay host MUST 3467 then inspect the list for any of the names or addresses by which it 3468 might be known in mail transactions. If a matching record is found, 3469 all records at that preference level and higher-numbered ones MUST be 3470 discarded from consideration. If there are no records left at that 3471 point, it is an error condition, and the message MUST be returned as 3472 undeliverable. If records do remain, they SHOULD be tried, best 3473 preference first, as described above. 3475 5.2. IPv6 and MX Records 3477 In the contemporary Internet, SMTP clients and servers may be hosted 3478 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3479 compatible with either version of the Internet Protocol. The host 3480 domains to which MX records point may, consequently, contain "A RR"s 3481 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3482 3974 [39] discusses some operational experience in mixed 3483 environments, it was not comprehensive enough to justify 3484 standardization, and some of its recommendations appear to be 3485 inconsistent with this specification. The appropriate actions to be 3486 taken either will depend on local circumstances, such as performance 3487 of the relevant networks and any conversions that might be necessary, 3488 or will be obvious (e.g., an IPv6-only client need not attempt to 3489 look up A RRs or attempt to reach IPv4-only servers). Designers of 3490 SMTP implementations that might run in IPv6 or dual-stack 3491 environments should study the procedures above, especially the 3492 comments about multihomed hosts, and, preferably, provide mechanisms 3493 to facilitate operational tuning and mail interoperability between 3494 IPv4 and IPv6 systems while considering local circumstances. 3496 6. Problem Detection and Handling 3498 6.1. Reliable Delivery and Replies by Email 3500 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3501 message in response to DATA), it is accepting responsibility for 3502 delivering or relaying the message. It must take this responsibility 3503 seriously. It MUST NOT lose the message for frivolous reasons, such 3504 as because the host later crashes or because of a predictable 3505 resource shortage. Some reasons that are not considered frivolous 3506 are discussed in the next subsection and in Section 7.8. 3508 If there is a delivery failure after acceptance of a message, the 3509 receiver-SMTP MUST formulate and mail a notification message. This 3510 notification MUST be sent using a null ("<>") reverse-path in the 3511 envelope. The recipient of this notification MUST be the address 3512 from the envelope return path (or the Return-Path: line). However, 3513 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3514 notification. Obviously, nothing in this section can or should 3515 prohibit local decisions (i.e., as part of the same system 3516 environment as the receiver-SMTP) to log or otherwise transmit 3517 information about null address events locally if that is desired. If 3518 the address is an explicit source route, it MUST be stripped down to 3519 its final hop. 3521 For example, suppose that an error notification must be sent for a 3522 message that arrived with: 3524 MAIL FROM:<@a,@b:user@d> 3526 The notification message MUST be sent using: 3528 RCPT TO: 3530 Some delivery failures after the message is accepted by SMTP will be 3531 unavoidable. For example, it may be impossible for the receiving 3532 SMTP server to validate all the delivery addresses in RCPT command(s) 3533 due to a "soft" domain system error, because the target is a mailing 3534 list (see earlier discussion of RCPT), or because the server is 3535 acting as a relay and has no immediate access to the delivering 3536 system. 3538 To avoid receiving duplicate messages as the result of timeouts, a 3539 receiver-SMTP MUST seek to minimize the time required to respond to 3540 the final . end of data indicator. See RFC 1047 [16] for 3541 a discussion of this problem. 3543 6.2. Unwanted, Unsolicited, and "Attack" Messages 3545 Utility and predictability of the Internet mail system requires that 3546 messages that can be delivered should be delivered, regardless of any 3547 syntax or other faults associated with those messages and regardless 3548 of their content. If they cannot be delivered, and cannot be 3549 rejected by the SMTP server during the SMTP transaction, they should 3550 be "bounced" (returned with non-delivery notification messages) as 3551 described above. In today's world, in which many SMTP server 3552 operators have discovered that the quantity of undesirable bulk email 3553 vastly exceeds the quantity of desired mail and in which accepting a 3554 message may trigger additional undesirable traffic by providing 3555 verification of the address, those principles may not be practical. 3557 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3558 without notification of the sender is permitted in practice. 3559 However, it is extremely dangerous and violates a long tradition and 3560 community expectations that mail is either delivered or returned. If 3561 silent message-dropping is misused, it could easily undermine 3562 confidence in the reliability of the Internet's mail systems. So 3563 silent dropping of messages should be considered only in those cases 3564 where there is very high confidence that the messages are seriously 3565 fraudulent or otherwise inappropriate. 3567 To stretch the principle of delivery if possible even further, it may 3568 be a rational policy to not deliver mail that has an invalid return 3569 address, although the history of the network is that users are 3570 typically better served by delivering any message that can be 3571 delivered. Reliably determining that a return address is invalid can 3572 be a difficult and time-consuming process, especially if the putative 3573 sending system is not directly accessible or does not fully and 3574 accurately support VRFY and, even if a "drop messages with invalid 3575 return addresses" policy is adopted, it SHOULD be applied only when 3576 there is near-certainty that the return addresses are, in fact, 3577 invalid. 3579 Conversely, if a message is rejected because it is found to contain 3580 hostile content (a decision that is outside the scope of an SMTP 3581 server as defined in this document), rejection ("bounce") messages 3582 SHOULD NOT be sent unless the receiving site is confident that those 3583 messages will be usefully delivered. The preference and default in 3584 these cases is to avoid sending non-delivery messages when the 3585 incoming message is determined to contain hostile content. 3587 6.3. Loop Detection 3589 Simple counting of the number of "Received:" header fields in a 3590 message has proven to be an effective, although rarely optimal, 3591 method of detecting loops in mail systems. SMTP servers using this 3592 technique SHOULD use a large rejection threshold, normally at least 3593 100 Received entries. Whatever mechanisms are used, servers MUST 3594 contain provisions for detecting and stopping trivial loops. 3596 6.4. Compensating for Irregularities 3598 Unfortunately, variations, creative interpretations, and outright 3599 violations of Internet mail protocols do occur; some would suggest 3600 that they occur quite frequently. The debate as to whether a well- 3601 behaved SMTP receiver or relay should reject a malformed message, 3602 attempt to pass it on unchanged, or attempt to repair it to increase 3603 the odds of successful delivery (or subsequent reply) began almost 3604 with the dawn of structured network mail and shows no signs of 3605 abating. Advocates of rejection claim that attempted repairs are 3606 rarely completely adequate and that rejection of bad messages is the 3607 only way to get the offending software repaired. Advocates of 3608 "repair" or "deliver no matter what" argue that users prefer that 3609 mail go through it if at all possible and that there are significant 3610 market pressures in that direction. In practice, these market 3611 pressures may be more important to particular vendors than strict 3612 conformance to the standards, regardless of the preference of the 3613 actual developers. 3615 The problems associated with ill-formed messages were exacerbated by 3616 the introduction of the split-UA mail reading protocols (Post Office 3617 Protocol (POP) version 2 [13], Post Office Protocol (POP) version 3 3618 [22], IMAP version 2 [18], and PCMAIL [17]). These protocols 3619 encouraged the use of SMTP as a posting (message submission) 3620 protocol, and SMTP servers as relay systems for these client hosts 3621 (which are often only intermittently connected to the Internet). 3622 Historically, many of those client machines lacked some of the 3623 mechanisms and information assumed by SMTP (and indeed, by the mail 3624 format protocol, RFC 822 [12]). Some could not keep adequate track 3625 of time; others had no concept of time zones; still others could not 3626 identify their own names or addresses; and, of course, none could 3627 satisfy the assumptions that underlay RFC 822's conception of 3628 authenticated addresses. 3630 In response to these weak SMTP clients, many SMTP systems now 3631 complete messages that are delivered to them in incomplete or 3632 incorrect form. This strategy is generally considered appropriate 3633 when the server can identify or authenticate the client, and there 3634 are prior agreements between them. By contrast, there is at best 3635 great concern about fixes applied by a relay or delivery SMTP server 3636 that has little or no knowledge of the user or client machine. Many 3637 of these issues are addressed by using a separate protocol, such as 3638 that defined in RFC 4409 [42], for message submission, rather than 3639 using originating SMTP servers for that purpose. 3641 The following changes to a message being processed MAY be applied 3642 when necessary by an originating SMTP server, or one used as the 3643 target of SMTP as an initial posting (message submission) protocol: 3645 o Addition of a message-id field when none appears 3647 o Addition of a date, time, or time zone when none appears 3649 o Correction of addresses to proper FQDN format 3651 The less information the server has about the client, the less likely 3652 these changes are to be correct and the more caution and conservatism 3653 should be applied when considering whether or not to perform fixes 3654 and how. These changes MUST NOT be applied by an SMTP server that 3655 provides an intermediate relay function. 3657 In all cases, properly operating clients supplying correct 3658 information are preferred to corrections by the SMTP server. In all 3659 cases, documentation SHOULD be provided in trace header fields and/or 3660 header field comments for actions performed by the servers. 3662 7. Security Considerations 3664 7.1. Mail Security and Spoofing 3666 SMTP mail is inherently insecure in that it is feasible for even 3667 fairly casual users to negotiate directly with receiving and relaying 3668 SMTP servers and create messages that will trick a naive recipient 3669 into believing that they came from somewhere else. Constructing such 3670 a message so that the "spoofed" behavior cannot be detected by an 3671 expert is somewhat more difficult, but not sufficiently so as to be a 3672 deterrent to someone who is determined and knowledgeable. 3673 Consequently, as knowledge of Internet mail increases, so does the 3674 knowledge that SMTP mail inherently cannot be authenticated, or 3675 integrity checks provided, at the transport level. Real mail 3676 security lies only in end-to-end methods involving the message 3677 bodies, such as those that use digital signatures (see RFC 1847 [20] 3678 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [45] or Secure/ 3679 Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [38]). 3681 Various protocol extensions and configuration options that provide 3682 authentication at the transport level (e.g., from an SMTP client to 3683 an SMTP server) improve somewhat on the traditional situation 3684 described above. However, in general, they only authenticate one 3685 server to another rather than a chain of relays and servers, much 3686 less authenticating users or user machines. Consequently, unless 3687 they are accompanied by careful handoffs of responsibility in a 3688 carefully designed trust environment, they remain inherently weaker 3689 than end-to-end mechanisms that use digitally signed messages rather 3690 than depending on the integrity of the transport system. 3692 Efforts to make it more difficult for users to set envelope return 3693 path and header "From" fields to point to valid addresses other than 3694 their own are largely misguided: they frustrate legitimate 3695 applications in which mail is sent by one user on behalf of another, 3696 in which error (or normal) replies should be directed to a special 3697 address, or in which a single message is sent to multiple recipients 3698 on different hosts. (Systems that provide convenient ways for users 3699 to alter these header fields on a per-message basis should attempt to 3700 establish a primary and permanent mailbox address for the user so 3701 that Sender header fields within the message data can be generated 3702 sensibly.) 3704 This specification does not further address the authentication issues 3705 associated with SMTP other than to advocate that useful functionality 3706 not be disabled in the hope of providing some small margin of 3707 protection against a user who is trying to fake mail. 3709 7.2. "Blind" Copies 3711 Addresses that do not appear in the message header section may appear 3712 in the RCPT commands to an SMTP server for a number of reasons. The 3713 two most common involve the use of a mailing address as a "list 3714 exploder" (a single address that resolves into multiple addresses) 3715 and the appearance of "blind copies". Especially when more than one 3716 RCPT command is present, and in order to avoid defeating some of the 3717 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3718 the full set of RCPT command arguments into the header section, 3719 either as part of trace header fields or as informational or private- 3720 extension header fields. [[CREF26: [rfc5321bis] [[Note in draft - 3721 Suggestion from 20070124 that got lost: delete "especially" and "the 3722 full set of" -- copying the first one can be as harmful as copying 3723 all of them, at least without verifying that the addresses do appear 3724 in the headers.]] Arnt Gulbrandsen, arnt@oryx.com, 2007.01.24 3725 1121+0100]] Since this rule is often violated in practice, and cannot 3726 be enforced, sending SMTP systems that are aware of "bcc" use MAY 3727 find it helpful to send each blind copy as a separate message 3728 transaction containing only a single RCPT command. 3730 There is no inherent relationship between either "reverse" (from the 3731 MAIL command) or "forward" (RCPT) addresses in the SMTP transaction 3732 ("envelope") and the addresses in the header section. Receiving 3733 systems SHOULD NOT attempt to deduce such relationships and use them 3734 to alter the header section of the message for delivery. The popular 3735 "Apparently-to" header field is a violation of this principle as well 3736 as a common source of unintended information disclosure and SHOULD 3737 NOT be used. 3739 7.3. VRFY, EXPN, and Security 3741 As discussed in Section 3.5, individual sites may want to disable 3742 either or both of VRFY or EXPN for security reasons (see below). As 3743 a corollary to the above, implementations that permit this MUST NOT 3744 appear to have verified addresses that are not, in fact, verified. 3745 If a site disables these commands for security reasons, the SMTP 3746 server MUST return a 252 response, rather than a code that could be 3747 confused with successful or unsuccessful verification. 3749 Returning a 250 reply code with the address listed in the VRFY 3750 command after having checked it only for syntax violates this rule. 3751 Of course, an implementation that "supports" VRFY by always returning 3752 550 whether or not the address is valid is equally not in 3753 conformance. 3755 On the public Internet, the contents of mailing lists have become 3756 popular as an address information source for so-called "spammers." 3757 The use of EXPN to "harvest" addresses has increased as list 3758 administrators have installed protections against inappropriate uses 3759 of the lists themselves. However, VRFY and EXPN are still useful for 3760 authenticated users and within an administrative domain. For 3761 example, VRFY and EXPN are useful for performing internal audits of 3762 how email gets routed to check and to make sure no one is 3763 automatically forwarding sensitive mail outside the organization. 3764 Sites implementing SMTP authentication may choose to make VRFY and 3765 EXPN available only to authenticated requestors. Implementations 3766 SHOULD still provide support for EXPN, but sites SHOULD carefully 3767 evaluate the tradeoffs. 3769 Whether disabling VRFY provides any real marginal security depends on 3770 a series of other conditions. In many cases, RCPT commands can be 3771 used to obtain the same information about address validity. On the 3772 other hand, especially in situations where determination of address 3773 validity for RCPT commands is deferred until after the DATA command 3774 is received, RCPT may return no information at all, while VRFY is 3775 expected to make a serious attempt to determine validity before 3776 generating a response code (see discussion above). 3778 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3780 Before a client uses the 251 or 551 reply codes from a RCPT command 3781 to automatically update its future behavior (e.g., updating the 3782 user's address book), it should be certain of the server's 3783 authenticity. If it does not, it may be subject to a man in the 3784 middle attack. 3786 7.5. Information Disclosure in Announcements 3788 There has been an ongoing debate about the tradeoffs between the 3789 debugging advantages of announcing server type and version (and, 3790 sometimes, even server domain name) in the greeting response or in 3791 response to the HELP command and the disadvantages of exposing 3792 information that might be useful in a potential hostile attack. The 3793 utility of the debugging information is beyond doubt. Those who 3794 argue for making it available point out that it is far better to 3795 actually secure an SMTP server rather than hope that trying to 3796 conceal known vulnerabilities by hiding the server's precise identity 3797 will provide more protection. Sites are encouraged to evaluate the 3798 tradeoff with that issue in mind; implementations SHOULD minimally 3799 provide for making type and version information available in some way 3800 to other network hosts. 3802 7.6. Information Disclosure in Trace Fields 3804 In some circumstances, such as when mail originates from within a LAN 3805 whose hosts are not directly on the public Internet, trace 3806 ("Received") header fields produced in conformance with this 3807 specification may disclose host names and similar information that 3808 would not normally be available. This ordinarily does not pose a 3809 problem, but sites with special concerns about name disclosure should 3810 be aware of it. Also, the optional FOR clause should be supplied 3811 with caution or not at all when multiple recipients are involved lest 3812 it inadvertently disclose the identities of "blind copy" recipients 3813 to others. 3815 7.7. Information Disclosure in Message Forwarding 3817 As discussed in Section 3.4, use of the 251 or 551 reply codes to 3818 identify the replacement address associated with a mailbox may 3819 inadvertently disclose sensitive information. Sites that are 3820 concerned about those issues should ensure that they select and 3821 configure servers appropriately. 3823 7.8. Resistance to Attacks 3825 In recent years, there has been an increase of attacks on SMTP 3826 servers, either in conjunction with attempts to discover addresses 3827 for sending unsolicited messages or simply to make the servers 3828 inaccessible to others (i.e., as an application-level denial of 3829 service attack). While the means of doing so are beyond the scope of 3830 this Standard, rational operational behavior requires that servers be 3831 permitted to detect such attacks and take action to defend 3832 themselves. For example, if a server determines that a large number 3833 of RCPT TO commands are being sent, most or all with invalid 3834 addresses, as part of such an attack, it would be reasonable for the 3835 server to close the connection after generating an appropriate number 3836 of 5yz (normally 550) replies. 3838 7.9. Scope of Operation of SMTP Servers 3840 It is a well-established principle that an SMTP server may refuse to 3841 accept mail for any operational or technical reason that makes sense 3842 to the site providing the server. However, cooperation among sites 3843 and installations makes the Internet possible. If sites take 3844 excessive advantage of the right to reject traffic, the ubiquity of 3845 email availability (one of the strengths of the Internet) will be 3846 threatened; considerable care should be taken and balance maintained 3847 if a site decides to be selective about the traffic it will accept 3848 and process. 3850 In recent years, use of the relay function through arbitrary sites 3851 has been used as part of hostile efforts to hide the actual origins 3852 of mail. Some sites have decided to limit the use of the relay 3853 function to known or identifiable sources, and implementations SHOULD 3854 provide the capability to perform this type of filtering. When mail 3855 is rejected for these or other policy reasons, a 550 code SHOULD be 3856 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3858 8. IANA Considerations 3860 IANA maintains three registries in support of this specification, all 3861 of which were created for RFC 2821 or earlier. This document expands 3862 the third one as specified below. The registry references listed are 3863 as of the time of publication; IANA does not guarantee the locations 3864 associated with the URLs. The registries are as follows: 3866 o The first, "Simple Mail Transfer Protocol (SMTP) Service 3867 Extensions" [49], consists of SMTP service extensions with the 3868 associated keywords, and, as needed, parameters and verbs. As 3869 specified in Section 2.2.2, no entry may be made in this registry 3870 that starts in an "X". Entries may be made only for service 3871 extensions (and associated keywords, parameters, or verbs) that 3872 are defined in Standards-Track or Experimental RFCs specifically 3873 approved by the IESG for this purpose. 3875 o The second registry, "Address Literal Tags" [50], consists of 3876 "tags" that identify forms of domain literals other than those for 3877 IPv4 addresses (specified in RFC 821 and in this document). The 3878 initial entry in that registry is for IPv6 addresses (specified in 3879 this document). Additional literal types require standardization 3880 before being used; none are anticipated at this time. 3882 o The third, "Mail Transmission Types" [49], established by RFC 821 3883 and renewed by this specification, is a registry of link and 3884 protocol identifiers to be used with the "via" and "with" 3885 subclauses of the time stamp ("Received:" header field) described 3886 in Section 4.4. Link and protocol identifiers in addition to 3887 those specified in this document may be registered only by 3888 standardization or by way of an RFC-documented, IESG-approved, 3889 Experimental protocol extension. This name space is for 3890 identification and not limited in size: the IESG is encouraged to 3891 approve on the basis of clear documentation and a distinct method 3892 rather than preferences about the properties of the method itself. 3894 An additional subsection has been added to the "VIA link types" 3895 and "WITH protocol types" subsections of this registry to contain 3896 registrations of "Additional-registered-clauses" as described 3897 above. The registry will contain clause names, a description, a 3898 summary of the syntax of the associated String, and a reference. 3899 As new clauses are defined, they may, in principle, specify 3900 creation of their own registries if the Strings consist of 3901 reserved terms or keywords rather than less restricted strings. 3902 As with link and protocol identifiers, additional clauses may be 3903 registered only by standardization or by way of an RFC-documented, 3904 IESG-approved, Experimental protocol extension. The additional 3905 clause name space is for identification and is not limited in 3906 size: the IESG is encouraged to approve on the basis of clear 3907 documentation, actual use or strong signs that the clause will be 3908 used, and a distinct requirement rather than preferences about the 3909 properties of the clause itself. 3911 In addition, if additional trace header fields (i.e., in addition to 3912 Return-path and Received) are ever created, those trace fields MUST 3913 be added to the IANA registry established by BCP 90 (RFC 3864) [8] 3914 for use with RFC 5322 [11]. 3916 9. Acknowledgments 3918 Many people contributed to the development of RFCs 2821 and 5321. 3919 Those documents should be consulted for those acknowledgments. 3921 Neither this document nor RFCs 2821 or 5321 would have been possible 3922 without the many contribution and insights of the late Jon Postel. 3923 Those contributions of course include the original specification of 3924 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3925 appears in this document as do several of Jon's original examples 3926 that have been updated only as needed to reflect other changes in the 3927 specification. 3929 The following filed errata against RFC 5321 that were not rejected at 3930 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3931 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3932 Rodrigo Speller, Alessandro Vesely, and Brett Watson. In addition, 3933 specific suggestions that led to corrections and improvements in 3934 early versions of the current specification were received from Ned 3935 Freed, Barry Leiba, Ivar Lumi, Pete Resnick, Hector Santos, Paul 3936 Smith and others. 3938 chetti contributed an analysis that clarified the ABNF productions 3939 that implicitly reference other documents. 3941 [[CREF27: Some errata and comments after 2019-07-01 have not yet been 3942 captured in this version of the draft. ]] 3944 The EMAILCORE Working Group was chartered in September 2020 with 3945 Alexey Melnikov and Seth Blank and co-chairs. Without their 3946 leadership and technical contributions, this document would never 3947 have been completed. 3949 10. References 3951 10.1. Normative References 3953 [1] Bradner, S., "Key words for use in RFCs to Indicate 3954 Requirement Levels", BCP 14, RFC 2119, 3955 DOI 10.17487/RFC2119, March 1997, 3956 . 3958 [2] American National Standards Institute (formerly United 3959 States of America Standards Institute), "USA Code for 3960 Information Interchange", ANSI X3.4-1968, 1968. 3962 ANSI X3.4-1968 has been replaced by newer versions with 3963 slight modifications, but the 1968 version remains 3964 definitive for the Internet. 3966 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 3967 RFC 821, DOI 10.17487/RFC0821, August 1982, 3968 . 3970 [4] Mockapetris, P., "Domain names - implementation and 3971 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3972 November 1987, . 3974 [5] Braden, R., Ed., "Requirements for Internet Hosts - 3975 Application and Support", STD 3, RFC 1123, 3976 DOI 10.17487/RFC1123, October 1989, 3977 . 3979 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 3980 Extension for Message Size Declaration", STD 10, RFC 1870, 3981 DOI 10.17487/RFC1870, November 1995, 3982 . 3984 [7] Newman, C., "ESMTP and LMTP Transmission Types 3985 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 3986 . 3988 [8] Klyne, G., Nottingham, M., and J. Mogul, "Registration 3989 Procedures for Message Header Fields", BCP 90, RFC 3864, 3990 DOI 10.17487/RFC3864, September 2004, 3991 . 3993 [9] Hinden, R. and S. Deering, "IP Version 6 Addressing 3994 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3995 2006, . 3997 [10] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 3998 Specifications: ABNF", STD 68, RFC 5234, 3999 DOI 10.17487/RFC5234, January 2008, 4000 . 4002 [11] Resnick, P., "Internet Message Format", RFC 5322, 4003 September 2008. 4005 10.2. Informative References 4007 [12] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 4008 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 4009 August 1982, . 4011 [13] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 4012 Reynolds, "Post Office Protocol: Version 2", RFC 937, 4013 DOI 10.17487/RFC0937, February 1985, 4014 . 4016 [14] Postel, J. and J. Reynolds, "File Transfer Protocol", 4017 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4018 . 4020 [15] Partridge, C., "Mail routing and the domain system", 4021 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4022 . 4024 [16] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4025 DOI 10.17487/RFC1047, February 1988, 4026 . 4028 [17] Lambert, M., "PCMAIL: A distributed mail system for 4029 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4030 1988, . 4032 [18] Crispin, M., "Interactive Mail Access Protocol: Version 4033 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4034 . 4036 [19] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4037 DOI 10.17487/RFC1846, September 1995, 4038 . 4040 [20] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4041 "Security Multiparts for MIME: Multipart/Signed and 4042 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4043 October 1995, . 4045 [21] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4046 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4047 DOI 10.17487/RFC1869, November 1995, 4048 . 4050 [22] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4051 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4052 . 4054 [23] De Winter, J., "SMTP Service Extension for Remote Message 4055 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4056 1996, . 4058 [24] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4059 Extensions (MIME) Part One: Format of Internet Message 4060 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4061 . 4063 [25] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4064 Part Three: Message Header Extensions for Non-ASCII Text", 4065 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4066 . 4068 [26] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4069 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4070 DOI 10.17487/RFC2156, January 1998, 4071 . 4073 [27] Elz, R. and R. Bush, "Clarifications to the DNS 4074 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4075 . 4077 [28] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4078 Word Extensions: Character Sets, Languages, and 4079 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4080 1997, . 4082 [29] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4083 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4084 . 4086 [30] Freed, N., "SMTP Service Extension for Command 4087 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4088 September 2000, . 4090 [31] Freed, N., "Behavior of and Requirements for Internet 4091 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4092 . 4094 [32] Vaudreuil, G., "SMTP Service Extensions for Transmission 4095 of Large and Binary MIME Messages", RFC 3030, 4096 DOI 10.17487/RFC3030, December 2000, 4097 . 4099 [33] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4100 Extension for Delivery Status Notifications (DSNs)", 4101 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4102 . 4104 [34] Vaudreuil, G., "Enhanced Mail System Status Codes", 4105 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4106 . 4108 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4109 for Delivery Status Notifications", RFC 3464, 4110 DOI 10.17487/RFC3464, January 2003, 4111 . 4113 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4114 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4115 . 4117 [37] Hansen, T., Ed. and G. Vaudreuil, Ed., "Message 4118 Disposition Notification", RFC 3798, DOI 10.17487/RFC3798, 4119 May 2004, . 4121 [38] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 4122 Extensions (S/MIME) Version 3.1 Message Specification", 4123 RFC 3851, DOI 10.17487/RFC3851, July 2004, 4124 . 4126 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4127 in Mixed IPv4/v6 Environments", RFC 3974, 4128 DOI 10.17487/RFC3974, January 2005, 4129 . 4131 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4132 Resource Identifier (URI): Generic Syntax", STD 66, 4133 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4134 . 4136 [41] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 4137 for Authorizing Use of Domains in E-Mail, Version 1", 4138 RFC 4408, DOI 10.17487/RFC4408, April 2006, 4139 . 4141 [42] Gellens, R. and J. Klensin, "Message Submission for Mail", 4142 RFC 4409, DOI 10.17487/RFC4409, April 2006, 4143 . 4145 [43] Fenton, J., "Analysis of Threats Motivating DomainKeys 4146 Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, 4147 September 2006, . 4149 [44] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 4150 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 4151 Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007, 4152 . 4154 [45] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4155 Thayer, "OpenPGP Message Format", RFC 4880, 4156 DOI 10.17487/RFC4880, November 2007, 4157 . 4159 [46] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4160 Mail System Status Codes", BCP 138, RFC 5248, 4161 DOI 10.17487/RFC5248, June 2008, 4162 . 4164 [47] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4165 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4166 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4167 . 4169 [48] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4170 DOI 10.17487/RFC7504, June 2015, 4171 . 4173 [49] Internet Assigned Number Authority (IANA), "IANA Mail 4174 Parameters", 2007, 4175 . 4177 [50] Internet Assigned Number Authority (IANA), "Address 4178 Literal Tags", 2007, 4179 . 4181 [51] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4182 Record for Domains that Accept No Mail", September 2014, 4183 . 4186 [52] RFC Editor, "RFC Errata - RFC 5321", 2019, 4187 . 4189 Captured 2019-11-19 4191 Appendix A. TCP Transport Service 4193 The TCP connection supports the transmission of 8-bit bytes. The 4194 SMTP data is 7-bit ASCII characters. Each character is transmitted 4195 as an 8-bit byte with the high-order bit cleared to zero. Service 4196 extensions may modify this rule to permit transmission of full 8-bit 4197 data bytes as part of the message body, or, if specifically designed 4198 to do so, in SMTP commands or responses. 4200 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4202 Some systems use an RFC 822 header section (only) in a mail 4203 submission protocol, or otherwise generate SMTP commands from RFC 822 4204 header fields when such a message is handed to an MTA from a UA. 4205 While the MTA-UA protocol is a private matter, not covered by any 4206 Internet Standard, there are problems with this approach. For 4207 example, there have been repeated problems with proper handling of 4208 "bcc" copies and redistribution lists when information that 4209 conceptually belongs to the mail envelope is not separated early in 4210 processing from header field information (and kept separate). 4212 It is recommended that the UA provide its initial ("submission 4213 client") MTA with an envelope separate from the message itself. 4214 However, if the envelope is not supplied, SMTP commands SHOULD be 4215 generated as follows: 4217 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4218 be copied to a RCPT command (generating multiple message copies 4219 if that is required for queuing or delivery). This includes any 4220 addresses listed in a RFC 822 "group". Any BCC header fields 4221 SHOULD then be removed from the header section. Once this 4222 process is completed, the remaining header fields SHOULD be 4223 checked to verify that at least one TO, CC, or BCC header field 4224 remains. If none do, then a BCC header field with no additional 4225 information SHOULD be inserted as specified in [11]. 4227 2. The return address in the MAIL command SHOULD, if possible, be 4228 derived from the system's identity for the submitting (local) 4229 user, and the "From:" header field otherwise. If there is a 4230 system identity available, it SHOULD also be copied to the Sender 4231 header field if it is different from the address in the From 4232 header field. (Any Sender header field that was already there 4233 SHOULD be removed.) Systems may provide a way for submitters to 4234 override the envelope return address, but may want to restrict 4235 its use to privileged users. This will not prevent mail forgery, 4236 but may lessen its incidence; see Section 7.1. 4238 When an MTA is being used in this way, it bears responsibility for 4239 ensuring that the message being transmitted is valid. The mechanisms 4240 for checking that validity, and for handling (or returning) messages 4241 that are not valid at the time of arrival, are part of the MUA-MTA 4242 interface and not covered by this specification. 4244 A submission protocol based on Standard RFC 822 information alone 4245 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4246 system into an SMTP environment. Additional information to construct 4247 an envelope must come from some source in the other environment, 4248 whether supplemental header fields or the foreign system's envelope. 4250 Attempts to gateway messages using only their header "To" and "Cc" 4251 fields have repeatedly caused mail loops and other behavior adverse 4252 to the proper functioning of the Internet mail environment. These 4253 problems have been especially common when the message originates from 4254 an Internet mailing list and is distributed into the foreign 4255 environment using envelope information. When these messages are then 4256 processed by a header-section-only remailer, loops back to the 4257 Internet environment (and the mailing list) are almost inevitable. 4259 Appendix C. Source Routes 4261 Historically, the was a reverse source routing list of 4262 hosts and a source mailbox. The first host in the was 4263 historically the host sending the MAIL command; today, source routes 4264 SHOULD NOT appear in the reverse-path. Similarly, the 4265 may be a source routing lists of hosts and a destination mailbox. 4266 However, in general, the SHOULD contain only a mailbox 4267 and domain name, relying on the domain name system to supply routing 4268 information if required. The use of source routes is deprecated (see 4269 Appendix F.2); while servers MUST be prepared to receive and handle 4270 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT 4271 transmit them and this section is included in the current 4272 specification only to provide context. It has been modified somewhat 4273 from the material in RFC 821 to prevent server actions that might 4274 confuse clients or subsequent servers that do not expect a full 4275 source route implementation. 4277 Historically, for relay purposes, the forward-path may have been a 4278 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4279 THREE MUST be fully-qualified domain names. This form was used to 4280 emphasize the distinction between an address and a route. The 4281 mailbox (here, JOE@THREE) is an absolute address, and the route is 4282 information about how to get there. The two concepts should not be 4283 confused.[[CREF28: [5321bis]JcK 20090123: Tightened this and the next 4284 paragraph to be clear that this doesn't authorize source route use.]] 4285 If source routes are used contrary to requirements and 4286 recommendations elsewhere in this specfiication, RFC 821 and the text 4287 below should be consulted for the mechanisms for constructing and 4288 updating the forward-path. A server that is reached by means of a 4289 source route (e.g., its domain name appears first in the list in the 4290 forward-path) MUST remove its domain name from any forward-paths in 4291 which that domain name appears before forwarding the message and MAY 4292 remove all other source routing information. The reverse-path SHOULD 4293 NOT be updated by servers conforming to this specification. 4295 Notice that the forward-path and reverse-path appear in the SMTP 4296 commands and replies, but not necessarily in the message. That is, 4297 there is no need for these paths and especially this syntax to appear 4298 in the "To:" , "From:", "CC:", etc. fields of the message header 4299 section. Conversely, SMTP servers MUST NOT derive final message 4300 routing information from message header fields. 4302 When the list of hosts is present despite the recommendations and 4303 requirements [[CREF29: [5321bis]JcK 20090123 "and requrements" 4304 added]] above, it is a "reverse" source route and indicates that the 4305 mail was relayed through each host on the list (the first host in the 4306 list was the most recent relay). This list is used as a source route 4307 to return non-delivery notices to the sender. If, contrary to the 4308 recommendations here, a relay host adds itself to the beginning of 4309 the list, it MUST use its name as known in the transport environment 4310 to which it is relaying the mail rather than that of the transport 4311 environment from which the mail came (if they are different). Note 4312 that a situation could easily arise in which some relay hosts add 4313 their names to the reverse source route and others do not, generating 4314 discontinuities in the routing list. This is another reason why 4315 servers needing to return a message SHOULD ignore the source route 4316 entirely and simply use the domain as specified in the Mailbox. 4318 Appendix D. Scenarios 4320 This section presents complete scenarios of several types of SMTP 4321 sessions. In the examples, "C:" indicates what is said by the SMTP 4322 client, and "S:" indicates what is said by the SMTP server. 4324 D.1. A Typical SMTP Transaction Scenario 4326 This SMTP example shows mail sent by Smith at host bar.com, and to 4327 Jones, Green, and Brown at host foo.com. Here we assume that host 4328 bar.com contacts host foo.com directly. The mail is accepted for 4329 Jones and Brown. Green does not have a mailbox at host foo.com. 4331 S: 220 foo.com Simple Mail Transfer Service Ready 4332 C: EHLO bar.com 4333 S: 250-foo.com greets bar.com 4334 S: 250-8BITMIME 4335 S: 250-SIZE 4336 S: 250-DSN 4337 S: 250 HELP 4338 C: MAIL FROM: 4339 S: 250 OK 4340 C: RCPT TO: 4341 S: 250 OK 4342 C: RCPT TO: 4343 S: 550 No such user here 4344 C: RCPT TO: 4345 S: 250 OK 4346 C: DATA 4347 S: 354 Start mail input; end with . 4348 C: Blah blah blah... 4349 C: ...etc. etc. etc. 4350 C: . 4351 S: 250 OK 4352 C: QUIT 4353 S: 221 foo.com Service closing transmission channel 4355 D.2. Aborted SMTP Transaction Scenario 4357 S: 220 foo.com Simple Mail Transfer Service Ready 4358 C: EHLO bar.com 4359 S: 250-foo.com greets bar.com 4360 S: 250-8BITMIME 4361 S: 250-SIZE 4362 S: 250-DSN 4363 S: 250 HELP 4364 C: MAIL FROM: 4365 S: 250 OK 4366 C: RCPT TO: 4367 S: 250 OK 4368 C: RCPT TO: 4369 S: 550 No such user here 4370 C: RSET 4371 S: 250 OK 4372 C: QUIT 4373 S: 221 foo.com Service closing transmission channel 4375 D.3. Relayed Mail Scenario 4377 Step 1 -- Source Host to Relay Host 4379 The source host performs a DNS lookup on XYZ.COM (the destination 4380 address) and finds DNS MX records specifying xyz.com as the best 4381 preference and foo.com as a lower preference. It attempts to open a 4382 connection to xyz.com and fails. It then opens a connection to 4383 foo.com, with the following dialogue: 4385 S: 220 foo.com Simple Mail Transfer Service Ready 4386 C: EHLO bar.com 4387 S: 250-foo.com greets bar.com 4388 S: 250-8BITMIME 4389 S: 250-SIZE 4390 S: 250-DSN 4391 S: 250 HELP 4392 C: MAIL FROM: 4393 S: 250 OK 4394 C: RCPT TO: 4395 S: 250 OK 4396 C: DATA 4397 S: 354 Start mail input; end with . 4398 C: Date: Thu, 21 May 1998 05:33:29 -0700 4399 C: From: John Q. Public 4400 C: Subject: The Next Meeting of the Board 4401 C: To: Jones@xyz.com 4402 C: 4403 C: Bill: 4404 C: The next meeting of the board of directors will be 4405 C: on Tuesday. 4406 C: John. 4407 C: . 4408 S: 250 OK 4409 C: QUIT 4410 S: 221 foo.com Service closing transmission channel 4412 Step 2 -- Relay Host to Destination Host 4414 foo.com, having received the message, now does a DNS lookup on 4415 xyz.com. It finds the same set of MX records, but cannot use the one 4416 that points to itself (or to any other host as a worse preference). 4417 It tries to open a connection to xyz.com itself and succeeds. Then 4418 we have: 4420 S: 220 xyz.com Simple Mail Transfer Service Ready 4421 C: EHLO foo.com 4422 S: 250 xyz.com is on the air 4423 C: MAIL FROM: 4424 S: 250 OK 4425 C: RCPT TO: 4426 S: 250 OK 4427 C: DATA 4428 S: 354 Start mail input; end with . 4429 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4430 C: 05:33:29 -0700 4431 C: Date: Thu, 21 May 1998 05:33:29 -0700 4432 C: From: John Q. Public 4433 C: Subject: The Next Meeting of the Board 4434 C: To: Jones@xyz.com 4435 C: 4436 C: Bill: 4437 C: The next meeting of the board of directors will be 4438 C: on Tuesday. 4439 C: John. 4440 C: . 4441 S: 250 OK 4442 C: QUIT 4443 S: 221 xyz.com Service closing transmission channel 4445 D.4. Verifying and Sending Scenario 4447 S: 220 foo.com Simple Mail Transfer Service Ready 4448 C: EHLO bar.com 4449 S: 250-foo.com greets bar.com 4450 S: 250-8BITMIME 4451 S: 250-SIZE 4452 S: 250-DSN 4453 S: 250-VRFY 4454 S: 250 HELP 4455 C: VRFY Crispin 4456 S: 250 Mark Crispin 4457 C: MAIL FROM: 4458 S: 250 OK 4459 C: RCPT TO: 4460 S: 250 OK 4461 C: DATA 4462 S: 354 Start mail input; end with . 4463 C: Blah blah blah... 4464 C: ...etc. etc. etc. 4465 C: . 4466 S: 250 OK 4467 C: QUIT 4468 S: 221 foo.com Service closing transmission channel 4470 Appendix E. Other Gateway Issues 4472 In general, gateways between the Internet and other mail systems 4473 SHOULD attempt to preserve any layering semantics across the 4474 boundaries between the two mail systems involved. Gateway- 4475 translation approaches that attempt to take shortcuts by mapping 4476 (such as mapping envelope information from one system to the message 4477 header section or body of another) have generally proven to be 4478 inadequate in important ways. Systems translating between 4479 environments that do not support both envelopes and a header section 4480 and Internet mail must be written with the understanding that some 4481 information loss is almost inevitable. 4483 Appendix F. Deprecated Features of RFC 821 4485 A few features of RFC 821 have proven to be problematic and SHOULD 4486 NOT be used in Internet mail. Some of these features were deprecated 4487 in RFC 2821 in 2001; source routing and two-digit years in dates were 4488 deprecated by RFC 1123 in 1989. Of the domain literal forms, RFC 4489 1123 required support only for the dotted decimal form. With the 4490 possible exception of old, hardware-embedded, applications, there is 4491 no longer any excuse for these features to appear on the contemporary 4492 Internet. [[CREF30: [5321bis] (2821ter) 2821bis Last Call Comment]] 4494 F.1. TURN 4496 This command, described in RFC 821, raises important security issues 4497 since, in the absence of strong authentication of the host requesting 4498 that the client and server switch roles, it can easily be used to 4499 divert mail from its correct destination. Its use is deprecated; 4500 SMTP systems SHOULD NOT use it unless the server can authenticate the 4501 client. 4503 F.2. Source Routing 4505 RFC 821 utilized the concept of explicit source routing to get mail 4506 from one host to another via a series of relays. The requirement to 4507 utilize source routes in regular mail traffic was eliminated by the 4508 introduction of the domain name system "MX" record and the last 4509 significant justification for them was eliminated by the 4510 introduction, in RFC 1123, of a clear requirement that addresses 4511 following an "@" must all be fully-qualified domain names. 4512 Consequently, the only remaining justifications for the use of source 4513 routes are support for very old SMTP clients or MUAs and in mail 4514 system debugging. They can, however, still be useful in the latter 4515 circumstance and for routing mail around serious, but temporary, 4516 problems such as problems with the relevant DNS records. 4518 SMTP servers MUST continue to accept source route syntax as specified 4519 in the main body of this document and in RFC 1123. They MAY, if 4520 necessary, ignore the routes and utilize only the target domain in 4521 the address. If they do utilize the source route, the message MUST 4522 be sent to the first domain shown in the address. In particular, a 4523 server MUST NOT guess at shortcuts within the source route. 4525 Clients SHOULD NOT utilize explicit source routing except under 4526 unusual circumstances, such as debugging or potentially relaying 4527 around firewall or mail system configuration errors. 4529 F.3. HELO 4531 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4532 than HELO when the server will accept the former. Servers MUST 4533 continue to accept and process HELO in order to support older 4534 clients. 4536 F.4. #-literals 4538 RFC 821 provided for specifying an Internet address as a decimal 4539 integer host number prefixed by a pound sign, "#". In practice, that 4540 form has been obsolete since the introduction of TCP/IP. It is 4541 deprecated and MUST NOT be used. 4543 F.5. Dates and Years 4545 When dates are inserted into messages by SMTP clients or servers 4546 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4547 digit years are deprecated; three-digit years were never permitted in 4548 the Internet mail system. 4550 F.6. Sending versus Mailing 4552 In addition to specifying a mechanism for delivering messages to 4553 user's mailboxes, RFC 821 provided additional, optional, commands to 4554 deliver messages directly to the user's terminal screen. These 4555 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4556 workstation technology and the introduction of other protocols may 4557 have rendered them obsolete even where they are implemented. 4559 [[5321bis Editor's Note: does this need a stronger reference to 821, 4560 2821, and/or 5321? Also, is anything else needed given the removal 4561 of these commands and comments about, e.g., their opening mail 4562 transaction sessions, from the mail body of the text?]] 4563 Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers 4564 MAY implement them. If they are implemented by servers, the 4565 implementation model specified in RFC 821 MUST be used and the 4566 command names MUST be published in the response to the EHLO command. 4568 Appendix G. Other Outstanding Issues 4570 [[RFC Editor: Please remove this section before publication.]] 4572 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4573 that led to a broad discussion of ways in which existing practice had 4574 diverged from the specifications and recommendations of RFC 5321 in 4575 the more than eleven years since it was published (some of those 4576 issues probably affect the boundary between RFC 5321 and 5322 and 4577 hence the latter as well). In most cases, those divergences call for 4578 revision of the Technical Specification to match the practice, 4579 clarification of the specification text in other ways, or a more 4580 comprehensive explanation of why the practices recommended by the 4581 specification should really be followed. 4583 Those discussions raised two other issues, which were that 4585 o The publication of the Submission Server specification of RFC 6409 4586 in November 2011 may not have been fully reflected in RFC 5321 4587 (despite the even earlier publication of RFC 4409) and 4589 o There may be inconsistencies between the July 2009 Internet Mail 4590 Architecture description of RFC 5598 and the model described in 4591 RFC 5321. The issue called out in Appendix G.3 below may be an 4592 example of one of those inconsistencies. 4594 Those discrepancies should be identified and discussed and decisions 4595 made to fix them (and where) or to ignore them and let them continue. 4597 There has also been discussion on the mailing list, perhaps amounting 4598 to very rough consensus, that any revision of RFC 5321 and/or 5322 4599 should be accompanied by a separate Applicability Statement document 4600 that would make recommendations about applicability or best practices 4601 in particular areas rather than trying to get everything into the two 4602 technical specifications. This appendix does not attempt to identify 4603 which issues should get which treatment. 4605 This work is now (starting in the last half of 2020) being considered 4606 in the EMAILCORE WG. This appendix will act as a temporary record of 4607 issues that should be discussed and decided upon before a revised 4608 SMTP specification (or a related Applicability Statement) is 4609 published, issues that have not been reflected in errata (see 4610 Appendix H.1 below for those covered by errata). 4612 Ticket numbers listed below reference the list in 4613 https://trac.ietf.org/trac/emailcore/report/1 . 4615 G.1. IP Address literals 4617 The specification is unclear about whether IP address literals, 4618 particularly IP address literals used as arguments to the EHLO 4619 command, are required to be accepted or whether they are allowed to 4620 be rejected as part of the general "operational necessity" exception. 4621 Some have suggested that rejection of them is so common as an anti- 4622 spam measure that the use of such literals should be deprecated 4623 entirely in the specification, others that the are still useful and 4624 used and/or that, whatever is said about IP address literals within 4625 an SMTP session (e.g., in MAIL or RCPT commands), they should 4626 continue to be allowed (and required) in EHLO. 4627 Ticket #1. 4629 G.2. Repeated Use of EHLO 4631 While the specification says that an SMTP client's sending EHLO again 4632 after it has been issued (starting an SMTP session and treats it as 4633 if RSET had been sent (closing the session) followed by EHLO, there 4634 are apparently applications, at least some of them involving setting 4635 up of secure connections, in which the second EHLO is required and 4636 does not imply RSET. Does the specification need to be adjusted to 4637 reflect or call out those cases? 4639 After extended discussion in October 2020, it appears that the 4640 easiest fix to these problems is to clarify the conditions for 4641 termination of a mail transaction in Section 3.3 and to clearly 4642 specify the effect of a second (or subsequent) EHLO command in 4643 Section 4.1.4. 4644 See also Appendix G.7.4. 4645 Ticket #2. Both changes have been made in draft-ietf-emailcore- 4646 rfc5321bis-01. 4648 G.3. Meaning of "MTA" and Related Terminology 4650 A terminology issue has come up about what the term "MTA" actually 4651 refers to, a question that became at least slightly more complicated 4652 when we formalized RFC 6409 Submission Servers. Does the document 4653 need to be adjusted to be more clear about this topic? Note that the 4654 answer may interact with the question asked in Section 2 above. 4655 Possibly along the same lines, RFC 2821 changed the RFC 821 4656 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4657 and "SMTP server" respectively. As things have evolved, it is 4658 possible that newer terminology is a source of confusion and that the 4659 terminology should be changed back, something that also needs 4660 discussion. 4661 Ticket #3. 4663 G.4. Originator, or Originating System, Authentication 4665 Should RFC 5321bis address authentication and related issues or 4666 should Section 3.9 or other text be reshaped (in addition to or 4667 instead of the comment on that section) to lay a better foundation 4668 for such work, either in the context of mailing lists or more 4669 generally? 4670 This may interact with Erratum 4055 and Ticket #30 below. 4672 G.5. Remove or deprecate the work-around from code 552 to 452 4674 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4675 and/or be inconsistent with current practice. Should it be removed 4676 and/or explicitly deprecated? 4677 Ticket #5. 4679 G.6. Clarify where the protocol stands with respect to submission and 4680 TLS issues 4682 1. submission on port 587 4684 2. submission on port 465 4686 3. TLS relay on a port different from 25 (whenever) 4688 4. Recommendations about general use of transport layer (hop by hop) 4689 security, particularly encryption including consideration of RFC 4690 8314. 4692 G.7. Probably-substantive Discussion Topics Identified in Other Ways 4694 The following issues were identified as a group in the opening Note 4695 but called out specifically only in embedded CREF comments in earlier 4696 (-00 and -01) versions of this draft. 4698 G.7.1. Issues with 521, 554, and 556 codes 4700 See new Section 4.2.4.2. More text may be needed, there or 4701 elsewhere, about choices of codes in response to initial opening and 4702 to EHLO, especially to deal with selective policy rejections. 4703 Ticket #6. 4705 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 4707 CREF comment in Section 2 and also CREF comment in Section 2.3.10 4709 G.7.3. Resolvable FQDNs and private domain names 4711 Multiple CREF comments in Section 2.3.5 4712 Tickets #9, #10 and #41. 4714 G.7.4. Possible clarification about mail transactions and transaction 4715 state 4717 CREF comment in Section 3.3 and also reference in Section 4.1.4 4718 Ticket #11. 4720 G.7.5. Issues with mailing lists, aliases, and forwarding 4722 CREF comment in Section 3.9. May also want to note forwarding as an 4723 email address portability issue. Note that, if changes are made in 4724 this area, they should be kept consistent with the description and 4725 discussion of the 251 and 551 in Section 4.2 and Section 3.5 as well 4726 as Section 3.4 to avoid introducing inconsistencies. In addition, 4727 there are some terminology issues about the use of the term "lists", 4728 identified in erratum 1820, that should be reviewed after any more 4729 substantive changes are made to the relevant sections. 4730 Ticket #12 and Ticket #34. 4732 G.7.6. Requirements for domain name and/or IP address in EHLO 4734 CREF comment in Section 4.1.4 4735 Ticket #19. 4737 G.7.7. Does the 'first digit only' and/or non-listed reply code text 4738 need clarification? 4740 CREF comments in Section 4.2 and Section 4.3.1 4741 Ticket #13. 4743 G.7.8. Size limits 4745 Once a decision is made about line length rules for RFC 5322bis, 4746 review the size limit discussions in this document, particularly the 4747 CREF comment (Note in Draft) at the end of the introductory material 4748 to Section 4.5.3 to be sure this document says what we want it to 4749 say. 4750 Ticket #14 and maybe Ticket #38. 4752 G.7.9. Discussion of 'blind' copies and RCPT 4754 CREF comment in Section 7.2. May also need to discussion whether 4755 that terminology is politically incorrect and suggest a replacement. 4756 Ticket #15. 4758 G.7.10. Further clarifications needed to source routes? 4760 The current text largely deprecates the use of source routes but 4761 suggests that servers continue to support them. Is additional work 4762 needed in this area? See CREF comment in Appendix C 4763 Ticket #17. 4765 G.7.11. Should 1yz Be Revisited? 4767 RFC 5321 depreciated the "positive preliminary reply" response code 4768 category with first digit "1", so that the first digit of valid SMTP 4769 response codes must be 2, 3, 4, or 5. It has been suggested (see 4770 mail from Hector Santos with Subject "SMTP Reply code 1yz Positive 4771 Preliminary reply", March 5, 2020 12:56 -0500, on the SMTP list) that 4772 these codes should be reinstated to deal with some situations that 4773 became more plausible after 5321 was published. Do we need to take 4774 this back up? 4775 Ticket #18. 4777 G.7.12. Review Timeout Specifications 4779 RFC 5321 (and its predecessors going back to 821) specify minimum 4780 periods for client and server to wait before timing out. Are those 4781 intervals still appropriate in a world of faster processors and 4782 faster networks? Should they be updated and revised? Or should more 4783 qualifying language be added? 4784 Ticket #16. 4786 G.7.13. Possible SEND, SAML, SOML Loose End 4788 Per discussion (and Ticket #20), the text about SEND, SAML, and SOML 4789 has been removed from the main body of the document so that the only 4790 discussion of them now appears in Appendix F.6. Per the editor's 4791 note in that appendix, is any further discussion needed? 4793 G.8. Enhanced Reply Codes and DSNs 4795 Enhanced Mail System Status Codes [34] were added to SMTP before RFC 4796 5321 was published and are now, together with a corresponding 4797 registry [46], widely deployed and in extensive use in the network. 4798 Similar, the structure and extensions options for Delivery Status 4799 Notifications [35] is implemented, deployed, and in wide use. Is it 4800 time to fold all or part of those mature specifications into the SMTP 4801 spec or at least to mention and normatively reference them? And, as 4802 an aside, do those specs need work or, if they are kept separate, is 4803 it time to move them to Internet Standard? 4805 G.9. Revisiting Quoted Strings 4807 Recent discussions both in and out of the IETF have highlighted 4808 instances of non-compliance with the specification of a Local-part 4809 consisting of a Quoted-string, whether any content of QcontentSMTP 4810 that actually requires special treatment consists of qtextSMTP, 4811 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4812 above) ends with a few paragraphs of warnings (essentially a partial 4813 applicability statement), the first of which cautions against 4814 cleverness with either Quoted-string or case sensitivity as a threat 4815 to interoperability. 4817 The Quoted-string portion of that discussion has apparently been 4818 widely not read or ignored. Do we need to do something else? If we 4819 do an Applicability Statement, would it be useful to either reference 4820 the discussion in this document from there or to move the discussion 4821 there and reference it (normatively?) from here? 4823 There has been a separate discussion of empty quoted strings in 4824 addresses, i.e., whether the production should be 4825 required to included at least one non-whitespace character. It is 4826 separate from this issue but would be further impacted or distorted 4827 from the considerations identified in this Section. 4828 Ticket #21. May also interact with Ticket #35. 4830 G.10. Internationalization 4832 RFC 5321 came long before work on internationalization of email 4833 addresses and headers (other than by use of encoded words in MINE) 4834 and specifically before the work of the EAI WG leading to the 4835 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4836 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4837 define mailboxes ...") is an extremely strong prohibition against the 4838 use of non-ASCII characters. Would it be appropriate to add 4839 something like "in the absence of relevant extensions" there? Also, 4840 given [mis]behavior seen in the wild, does that paragraph (or an A/S) 4841 need an explicit caution about SMTP servers or clients assuming they 4842 can apply the popular web convention of using %NN sequences as a way 4843 to encode non-ASCII characters ( in RFC 3986) and 4844 assuming some later system will interpret it as they expect? Would 4845 it be appropriate to add an Internationalization Considerations 4846 section to the body of this document if only for the purpose of 4847 pointing people elsewhere? 4849 G.11. SMTP Clients, Servers, Senders, and Receivers 4851 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4852 2821 (and hence in 5321), we switched that to "client" and "server" 4853 (See the discussion in Section 1.2). In part because a relay is a 4854 server and then a client (in some recent practice, even interleaving 4855 the two functions by opening the connection to the next host in line 4856 and sending commands before the incoming transaction is complete), 4857 RFC 5321 continues to use the original terminology in some places. 4858 Should we revisit that usage, possibly even returning to consistent 4859 use of the original terminology? 4861 G.12. Extension Keywords Starting in 'X-' 4863 Section 2.2.2 contains a discussion of SMTP keywords starting in "X". 4864 Given general experience with such things and RFC 6648, is there any 4865 reason to not deprecate that practice entirely and remove that text? 4866 If we do so, should Section 4.1.5 be dropped or rewritten to make 4867 clear this is an obsolete practice? 4868 Ticket #42. 4870 G.13. Deprecating HELO 4872 RFC 5321 (and 2821 before it) very carefully circle around the status 4873 of HELO, even recommending its use as a fallback when EHLO is sent 4874 and a "command not recognized" response is received. We are just a 4875 few months short of 20 years; is it time to deprecate the thing and 4876 clean out some or all of that text? And, given a recent (4Q2020) 4877 discussion on the EMAILCORE list, should EHLO be explicitly bound to 4878 SMTP over TCP with the older transports allowed only with HELO? 4879 While those questions may seem independent, separating them is fairly 4880 hard given the way the text is now constructed. 4881 Ticket #43. 4883 Appendix H. RFC 5321 Errata Summary and Tentative Change Log 4885 [[RFC Editor: Please remove this section before publication.]] 4887 H.1. RFC 5321 Errata Summary 4889 This document addresses the following errata filed against RFC 5321 4890 since its publication in October 2008 [52]. As with the previous 4891 appendix, ticket numbers included below reference 4892 https://trac.ietf.org/trac/emailcore/report/1 . [[CREF31: [[Note in 4893 Draft: Items with comments below have not yet been resolved as 4894 errata. As of the end of November 2020, none of them have been 4895 checked and verified by the emailcore WG.]]]]. 4897 1683 ABNF error. Section 4.4 4898 Ticket #23. 4900 4198 Description error. Section 4.2. 4901 RESOLVED, ticket #24, 2020-12-14. 4903 2578 Syntax description error. Section 4.1.2 4905 1543 Wrong code in description Section 3.8 4906 Ticket #26 4908 4315 ABNF - IPv6 Section 4.1.3. [[CREF32: [5321bis]The IPv6 syntax 4909 has been adjusted since 5321 was published. See the rewritten 4910 form and the comment in the section cited in the previous 4911 sentence. The editor awaits instructions. See https://www.rfc- 4912 editor.org/errata/eid4315]] 4913 Ticket #27. 4915 5414 ABNF for Quoted-string Section 4.1.2 4916 Ticket #22. 4918 1851 Location of text on unexpected close Section 4.1.1.5. 4919 [[CREF33: [5321bis]Matter of taste, editor seeks advice.]] 4920 Ticket #28. 4922 3447 Use of normative language (e.g., more "MUST"s), possible 4923 confusion in some sections Section 4.4. [[CREF34: [5321bis]As 4924 Barry notes in his verifier comments on the erratum (see 4925 https://www.rfc-editor.org/errata/eid3447), the comments and 4926 suggestions here raise a number of interesting (and difficult) 4927 issues. One of the issues is that the core of RFCs 5321 (and 4928 2821) is text carried over from Jon Postel's RFC 821, a document 4929 that was not only written in a different style than the IETF uses 4930 today but that was written at a time when no one had dreamt of RFC 4931 2119 or even the IETF itself. It appears to me that trying to 4932 patch that style might easily result in a document that is harder 4933 to read as well as being error prone. If we want to get the 4934 document entirely into contemporary style, we really should bite 4935 the bullet and do a complete rewrite. To respond to a different 4936 point in Barry's discussion, I think an explicit statement that 4937 5321/5322 and their predecessors differ in places and why would be 4938 helpful. Text, and suggestions about where to put it, are 4939 solicited. A list of differences might be a good idea too, but 4940 getting it right might be more work than there is available energy 4941 to do correctly. ]] 4943 5711 Missing leading spaces in example Appendix D.3. [[CREF35: 4944 [5321bis]Well, this is interesting because the XML is correct and 4945 the spaces are there, embedded in artwork. So either the XML2RFC 4946 processor at the time took those leading spaces out or the RFC 4947 Editor improved on the document and the change was not caught in 4948 AUTH48, perhaps because rfcdiff ignores white space. We just need 4949 to watch for future iterations. ]] 4950 Ticket #29. 4952 4055 Erratum claims the the description of SPF and DKIM is wrong. 4953 It is not clear what 5321bis should really say about them, but the 4954 current text probably needs work (or dropping, which is what the 4955 proposed erratum suggests). See 5321bis Editor's Note in 4956 Section 3.6.2. 4957 Ticket #30. 4959 [[CREF36: [5321bis]Note that rejected errata have _not_ been reviewed 4960 to see if they contain anything useful that should be discussed again 4961 with the possibility of rethinking and changing text. Volunteers 4962 sought.]] 4964 H.2. Changes from RFC 5321 (published October 2008) to the initial 4965 (-00) version of this draft 4967 o Acknowledgments section (Section 9) trimmed back for new document. 4969 o Introductory paragraph to Appendix F extended to make it clear 4970 that these features were deprecated a long time ago and really 4971 should not be in use any more. 4973 o Adjusted some language to clarify that source routes really, 4974 really, should not be used or depended upon. 4976 o IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 4977 a note added. 4979 o Production index added as a first step in tying all productions to 4980 their sources. As part of the effort to make the document more 4981 easily navigable, table of contents entries have been created for 4982 the individual command descriptions. 4984 o Clarified the relationship between the SMTP "letters, digits, and 4985 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 4987 o Revised the reply code sections to add new 521 and 556 codes, 4988 clarify relationships, and be explicit about the requirement for 4989 clients to rely on first digits rather than the sequences in 4990 Section 4.3.2. 4992 o In conjunction with the above, explicitly obsolete RFCs 1846 and 4993 7504. 4995 o Incorporated a correction reflecting Errata ID 2578. 4997 o Some small editorial changes made to eliminate redundant 4998 statements that were very close together. Other, equally small, 4999 editorial changes have been made to improve grammar or clarity. 5001 o A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 5002 Draft" have been added for the group to resolve. Other questions, 5003 especially those in the errata summary, are simply included in 5004 narrative comments in CREFs. 5006 o Checked and rationalized "response" (to a command) and "reply 5007 code" terminology. One can talk about a "999 response" but only a 5008 "999 reply code". There is no such thing as a "response code". 5010 o Added note about length limit on mailbox names ("email 5011 addresses"). 5013 o Added an "errata summary" subsection to this change log/ 5014 comparison to 5321 in this Appendix. The entire Appendix will, of 5015 course, disappear at the time of RFC publication unless someone 5016 wants to make a strong case for retaining it. 5018 o Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 5019 readers below the Abstract. 5021 o Temporarily added a "Note on Reading This Working Draft" after the 5022 Abstract. 5024 H.3. Changes Among Versions of Rfc5321bis 5026 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 5027 -01 5029 Substantively, these two versions differ only by suppression of the 5030 CREF and other discussion associated with the evolution from RFC 2821 5031 to RFC 5321. That change includes an update to the document's Note 5032 to Readers, the date, the file name, and the addition of this change 5033 log subsection. 5035 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 5037 o Minor clarifications to improve text, e.g., addition of NOOP to 5038 the list of non-mail transaction examples in Section 4.1.4. 5040 o Added topics exposed in the ietf-smtp list and the IETF list 5041 "dogfood" discussion during December 2019 and an index listing of 5042 substantive issues identified only in CREFs in the prior draft as 5043 a new Appendix G.. 5045 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 5047 o Added more text to Appendix G.7.1 to specifically call out the 5048 session-opening policy issues surrounding these codes. 5050 o Added discussion of "1yz" reinstatement in Appendix G.7.11. 5052 o Added discussion of timeouts in Appendix G.7.12. 5054 o Added subsection on Enhanced Status Codes and DSNs to the 5055 outstanding issues list Appendix G.8. 5057 o Replaced reference to RFC 1652 (8BITMIME) with the Internet 5058 Standard version, RFC 6152. 5060 o With help from cketti, clarified the ABNF productions whose 5061 terminals appear in other documents. 5063 o Added discussions of Quoted-string, Internationalization, and 5064 client-server versus sender-receiver terminology to Appendix G. 5066 o Added note to the Abstract. 5068 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) to draft- 5069 ietf-emailcore-rfc5321bis-00 5071 o Added a paragraph about non-null quoted strings to Appendix G.9. 5073 o Added an explicit pointer to email insecurity and TLS to 5074 Appendix G.6. Inspired by Ben Kaduk's comment on the WG Charter, 5075 2020-09-09. 5077 o Converted document from individual to emailcore WG effort. 5079 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 (2020-10-06) to 5080 -01 5082 o Editorial: Corrected "blackslash" to "backslash" 5084 o Rewrote the introduction to Appendix G slightly to reflect the 5085 creation of the EMAILCORE WG. 5087 o Applied fixes for repeated use of EHLO. See Appendix G.2. 5089 o Added two new questions, one about "X" extensions (Appendix G.12) 5090 and one about the status of HELO (Appendix G.13). 5092 o Removed mention of SEND, SAML, SOML from the main body of the text 5093 (Ticket #20). 5095 o Added a warning about side effects to Appendix G.7.5. 5097 o Added ticket numbers to descriptions of issues and changes, 5098 adjusted some text so relationships would be more clear, and added 5099 subsections to the Appendix G and H lists to pick up on tickets 5100 that were not easily identified in those sections of with the 5101 text. 5103 o Made several additions to the Index, including one to deal with 5104 SEND et al., as above. 5106 Index 5108 A 5109 Argument Syntax 5110 A-d-l 42 5111 Additional-Registered-Clauses 63 5112 address-literal 43 5113 Addtl-Link 63 5114 Addtl-Protocol 63 5115 ALPHA 42 5116 Argument 43 5117 At-domain 42 5118 atext 42 5119 Atom 43 5120 By-domain 62 5121 CFWS 42 5122 CRLF 42 5123 dcontent 45 5124 DIGIT 42 5125 Domain 43 5126 Dot-string 43 5127 esmtp-keyword 42 5128 esmtp-param 42 5129 esmtp-value 43 5130 Extended-Domain 62 5131 For 63 5132 Forward-Path 42 5133 From-domain 62 5134 FWS 42 5135 General-address-literal 45 5136 Greeting 48 5137 h16 45 5138 HEXDIG 42 5139 ID 63 5140 IPv4-address-literal 45 5141 IPv6-addr 45 5142 IPv6-address-literal 45 5143 Keyword 43 5144 Ldh-str 43 5145 Let-dig 43 5146 Link 63 5147 Local-part 43 5148 ls32 45 5149 Mail-parameters 42 5150 Mailbox 43 5151 Opt-info 63 5152 Path 42 5153 Protocol 63 5154 QcontentSMTP 43 5155 qtextSMTP 43 5156 quoted-pairSMTP 43 5157 Quoted-string 43 5158 Rcpt-parameters 42 5159 Reply-code 48 5160 Reply-line 48 5161 Return-path-line 62 5162 Reverse-Path 42 5163 Snum 45 5164 SP 42 5165 Stamp 62 5166 Standardized-tag 45 5167 String 43 5168 sub-domain 43 5169 TCP-info 62 5170 textstring 48 5171 Time-stamp-line 62 5172 Via 63 5173 With 63 5175 C 5176 Command Syntax 5177 data 39 5178 ehlo 20, 34 5179 expn 40 5180 helo 34 5181 help 41 5182 mail 36 5183 noop 41 5184 quit 41 5185 rcpt 38 5186 rset 40 5187 send, saml, soml 102 5188 vrfy 40 5190 Author's Address 5192 John C. Klensin 5193 1770 Massachusetts Ave, Suite 322 5194 Cambridge, MA 02140 5195 USA 5197 EMail: john-ietf@jck.com