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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 October 6, 2020 4 Obsoletes: 5321, 1846, 7504 (if 5 approved) 6 Updates: 1123 (if approved) 7 Intended status: Standards Track 8 Expires: April 9, 2021 10 Simple Mail Transfer Protocol 11 draft-ietf-emailcore-rfc5321bis-00 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 April 9, 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 . . . . . . . . . . . . . . . . . . . . 12 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 . . . . . . 26 118 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 . . . . . . . . 28 123 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 29 124 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 30 125 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 30 126 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 30 127 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 . . . . . . . . . . . . . . . . . . 45 139 4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . 47 140 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 47 141 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 . . . . . . . . . . . . . . . . 71 158 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 73 159 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 . . . . . . . . . . . . . . . . . . . 77 165 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 77 166 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 78 167 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 . . . . . . . . . 80 172 7.7. Information Disclosure in Message Forwarding . . . . . . 80 173 7.8. Resistance to Attacks . . . . . . . . . . . . . . . . . . 81 174 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 81 175 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 81 176 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 83 177 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 83 178 10.1. Normative References . . . . . . . . . . . . . . . . . . 83 179 10.2. Informative References . . . . . . . . . . . . . . . . . 84 180 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 89 181 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 89 182 Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . 90 183 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 91 184 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 91 185 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 92 186 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 93 187 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 94 188 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 95 189 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 95 190 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 95 191 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 95 192 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 96 193 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 96 194 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 96 195 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 96 196 Appendix G. Other Outstanding Issues . . . . . . . . . . . . . . 97 197 G.1. IP Address Literals . . . . . . . . . . . . . . . . . . . 98 198 G.2. Repeated Use of EHLO . . . . . . . . . . . . . . . . . . 98 199 G.3. Meaning of "MTA" and Related Terminology . . . . . . . . 98 200 G.4. Originator, or Originating System, Authentication . . . . 98 201 G.5. Remove or deprecate the work-around from code 552 to 452 99 202 G.6. Clarify where the protocol stands with respect to 203 submission and TLS issues . . . . . . . . . . . . . . . . 99 204 G.7. Probably-substantive Discussion Topics Identified in 205 Other Ways . . . . . . . . . . . . . . . . . . . . . . . 99 206 G.7.1. Issues with 521, 554, and 556 codes . . . . . . . . . 99 207 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 99 208 G.7.3. Resolvable FQDNs and private domain names . . . . . . 99 209 G.7.4. Possible clarification about mail transactions and 210 transaction state . . . . . . . . . . . . . . . . . . 99 211 G.7.5. Issues with mailing lists, aliases, and forwarding . 100 212 G.7.6. Requirements for domain name and/or IP address in 213 EHLO . . . . . . . . . . . . . . . . . . . . . . . . 100 214 G.7.7. Does the 'first digit only' and/or non-listed reply 215 code text need clarification? . . . . . . . . . . . . 100 216 G.7.8. Size limits . . . . . . . . . . . . . . . . . . . . . 100 217 G.7.9. Discussion of 'blind' copies and RCPT . . . . . . . . 100 218 G.7.10. Further clarifications needed to source routes? . . . 100 219 G.7.11. Should 1yz Be Revisited? . . . . . . . . . . . . . . 100 220 G.7.12. Review Timeout Specifications . . . . . . . . . . . . 100 221 G.8. Enhanced Reply Codes and DSNs . . . . . . . . . . . . . . 101 222 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 101 223 G.10. Internationalization . . . . . . . . . . . . . . . . . . 101 224 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 102 225 Appendix H. Change log for RFC 5321bis . . . . . . . . . . . . . 102 226 H.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 102 227 H.2. Changes from RFC 5321 (published October 2008) to the 228 initial (-00) version of this draft . . . . . . . . . . . 103 229 H.3. Changes Among Versions of Rfc5321bis . . . . . . . . . . 105 230 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 231 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 105 232 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) 233 to -02 . . . . . . . . . . . . . . . . . . . . . . . 105 234 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 235 to -03 . . . . . . . . . . . . . . . . . . . . . . . 105 236 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) 237 to draft-ietf-emailcore-rfc5321bis-00 . . . . . . . . 106 238 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 239 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 108 241 1. Introduction 243 1.1. Transport of Electronic Mail 245 The objective of the Simple Mail Transfer Protocol (SMTP) is to 246 transfer mail reliably and efficiently. 248 SMTP is independent of the particular transmission subsystem and 249 requires only a reliable ordered data stream channel. While this 250 document specifically discusses transport over TCP, other transports 251 are possible. Appendices to RFC 821 [3] describe some of them. 253 An important feature of SMTP is its capability to transport mail 254 across multiple networks, usually referred to as "SMTP mail relaying" 255 (see Section 3.6). A network consists of the mutually-TCP-accessible 256 hosts on the public Internet, the mutually-TCP-accessible hosts on a 257 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 258 environment utilizing a non-TCP transport-level protocol. Using 259 SMTP, a process can transfer mail to another process on the same 260 network or to some other network via a relay or gateway process 261 accessible to both networks. 263 In this way, a mail message may pass through a number of intermediate 264 relay or gateway hosts on its path from sender to ultimate recipient. 265 The Mail eXchanger mechanisms of the domain name system (RFC 1035 266 [4], RFC 974 [15], and Section 5 of this document) are used to 267 identify the appropriate next-hop destination for a message being 268 transported. 270 1.2. History and Context for This Document 272 This document is a specification of the basic protocol for the 273 Internet electronic mail transport. It consolidates, updates and 274 clarifies, but does not add new or change existing functionality of 275 the following: 277 o the original SMTP (Simple Mail Transfer Protocol) specification of 278 RFC 821 [3], 280 o domain name system requirements and implications for mail 281 transport from RFC 1035 [4] and RFC 974 [15], 283 o the clarifications and applicability statements in RFC 1123 [5], 285 o the new error codes added by RFC 1846 [19] and later by RFC 7504 286 [48], obsoleting both of those documents, and 288 o material drawn from the SMTP Extension mechanisms in RFC 1869 289 [21]. 291 o Editorial and clarification changes to RFC 2821 [29] to bring that 292 specification to Draft Standard. 294 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC 295 1123 (replacing the mail transport materials of RFC 1123). However, 296 RFC 821 specifies some features that were not in significant use in 297 the Internet by the mid-1990s and (in appendices) some additional 298 transport models. Those sections are omitted here in the interest of 299 clarity and brevity; readers needing them should refer to RFC 821. 301 It also includes some additional material from RFC 1123 that required 302 amplification. This material has been identified in multiple ways, 303 mostly by tracking flaming on various lists and newsgroups and 304 problems of unusual readings or interpretations that have appeared as 305 the SMTP extensions have been deployed. Where this specification 306 moves beyond consolidation and actually differs from earlier 307 documents, it supersedes them technically as well as textually. 309 Although SMTP was designed as a mail transport and delivery protocol, 310 this specification also contains information that is important to its 311 use as a "mail submission" protocol, as recommended for Post Office 312 Protocol (POP) (RFC 937 [13], RFC 1939 [22]) and IMAP (RFC 3501 313 [36]). In general, the separate mail submission protocol specified 314 in RFC 4409 [42] is now preferred to direct use of SMTP; more 315 discussion of that subject appears in that document. 317 Section 2.3 provides definitions of terms specific to this document. 318 Except when the historical terminology is necessary for clarity, this 319 document uses the current 'client' and 'server' terminology to 320 identify the sending and receiving SMTP processes, respectively. 322 A companion document, RFC 5322 [11], discusses message header 323 sections and bodies and specifies formats and structures for them. 325 1.3. Document Conventions 327 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 328 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 329 document are to be interpreted as described in RFC 2119 [1]. As each 330 of these terms was intentionally and carefully chosen to improve the 331 interoperability of email, each use of these terms is to be treated 332 as a conformance requirement. 334 Because this document has a long history and to avoid the risk of 335 various errors and of confusing readers and documents that point to 336 this one, most examples and the domain names they contain are 337 preserved from RFC 2821. Readers are cautioned that these are 338 illustrative examples that should not actually be used in either code 339 or configuration files. 341 2. The SMTP Model 343 [[CREF2: [5321bis] [[Editor's Note: There have been extensive and 344 repeated discussions on the SMTP and IETF lists about whether this 345 document should say something about hop-by-hop (MTA-to-MTA) SMTP 346 authentication and, if so, what?? Note that end to end message 347 authentication is almost certainly out of scope for SMTP.]]]] 349 2.1. Basic Structure 351 The SMTP design can be pictured as: 353 +----------+ +----------+ 354 +------+ | | | | 355 | User |<-->| | SMTP | | 356 +------+ | Client- |Commands/Replies| Server- | 357 +------+ | SMTP |<-------------->| SMTP | +------+ 358 | File |<-->| | and Mail | |<-->| File | 359 |System| | | | | |System| 360 +------+ +----------+ +----------+ +------+ 361 SMTP client SMTP server 363 When an SMTP client has a message to transmit, it establishes a two- 364 way transmission channel to an SMTP server. The responsibility of an 365 SMTP client is to transfer mail messages to one or more SMTP servers, 366 or report its failure to do so. 368 The means by which a mail message is presented to an SMTP client, and 369 how that client determines the identifier(s) ("names") of the 370 domain(s) to which mail messages are to be transferred, are local 371 matters. They are not addressed by this document. In some cases, 372 the designated domain(s), or those determined by an SMTP client, will 373 identify the final destination(s) of the mail message. In other 374 cases, common with SMTP clients associated with implementations of 375 the POP (RFC 937 [13], RFC 1939 [22]) or IMAP (RFC 3501 [36]) 376 protocols, or when the SMTP client is inside an isolated transport 377 service environment, the domain determined will identify an 378 intermediate destination through which all mail messages are to be 379 relayed. SMTP clients that transfer all traffic regardless of the 380 target domains associated with the individual messages, or that do 381 not maintain queues for retrying message transmissions that initially 382 cannot be completed, may otherwise conform to this specification but 383 are not considered fully-capable. Fully-capable SMTP 384 implementations, including the relays used by these less capable 385 ones, and their destinations, are expected to support all of the 386 queuing, retrying, and alternate address functions discussed in this 387 specification. In many situations and configurations, the less- 388 capable clients discussed above SHOULD be using the message 389 submission protocol (RFC 4409 [42]) rather than SMTP. 391 The means by which an SMTP client, once it has determined a target 392 domain, determines the identity of an SMTP server to which a copy of 393 a message is to be transferred, and then performs that transfer, are 394 covered by this document. To effect a mail transfer to an SMTP 395 server, an SMTP client establishes a two-way transmission channel to 396 that SMTP server. An SMTP client determines the address of an 397 appropriate host running an SMTP server by resolving a destination 398 domain name to either an intermediate Mail eXchanger host or a final 399 target host. 401 An SMTP server may be either the ultimate destination or an 402 intermediate "relay" (that is, it may assume the role of an SMTP 403 client after receiving the message) or "gateway" (that is, it may 404 transport the message further using some protocol other than SMTP). 405 SMTP commands are generated by the SMTP client and sent to the SMTP 406 server. SMTP replies are sent from the SMTP server to the SMTP 407 client in response to the commands. 409 In other words, message transfer can occur in a single connection 410 between the original SMTP-sender and the final SMTP-recipient, or can 411 occur in a series of hops through intermediary systems. In either 412 case, once the server has issued a success response at the end of the 413 mail data, a formal handoff of responsibility for the message occurs: 414 the protocol requires that a server MUST accept responsibility for 415 either delivering the message or properly reporting the failure to do 416 so (see Sections 6.1, 6.2, and 7.8, below). 418 Once the transmission channel is established and initial handshaking 419 is completed, the SMTP client normally initiates a mail transaction. 420 Such a transaction consists of a series of commands to specify the 421 originator and destination of the mail and transmission of the 422 message content (including any lines in the header section or other 423 structure) itself. When the same message is sent to multiple 424 recipients, this protocol encourages the transmission of only one 425 copy of the data for all recipients at the same destination (or 426 intermediate relay) host. 428 The server responds to each command with a reply; replies may 429 indicate that the command was accepted, that additional commands are 430 expected, or that a temporary or permanent error condition exists. 431 Commands specifying the sender or recipients may include server- 432 permitted SMTP service extension requests, as discussed in 433 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 434 although this can be modified by mutually agreed upon extension 435 requests such as command pipelining (RFC 2920 [30]). 437 Once a given mail message has been transmitted, the client may either 438 request that the connection be shut down or may initiate other mail 439 transactions. In addition, an SMTP client may use a connection to an 440 SMTP server for ancillary services such as verification of email 441 addresses or retrieval of mailing list subscriber addresses. 443 As suggested above, this protocol provides mechanisms for the 444 transmission of mail. Historically, this transmission normally 445 occurred directly from the sending user's host to the receiving 446 user's host when the two hosts are connected to the same transport 447 service. When they are not connected to the same transport service, 448 transmission occurs via one or more relay SMTP servers. A very 449 common case in the Internet today involves submission of the original 450 message to an intermediate, "message submission" server, which is 451 similar to a relay but has some additional properties; such servers 452 are discussed in Section 2.3.10 and at some length in RFC 4409 [42]. 453 An intermediate host that acts as either an SMTP relay or as a 454 gateway into some other transmission environment is usually selected 455 through the use of the domain name service (DNS) Mail eXchanger 456 mechanism. Explicit "source" routing (see Section 5 and Appendix C 457 and Appendix F.2) SHOULD NOT be used. [[CREF3: [5321bis] JcK 458 20090123 - redundant sentence removed.]] 460 2.2. The Extension Model 462 2.2.1. Background 464 In an effort that started in 1990, approximately a decade after RFC 465 821 was completed, the protocol was modified with a "service 466 extensions" model that permits the client and server to agree to 467 utilize shared functionality beyond the original SMTP requirements. 468 The SMTP extension mechanism defines a means whereby an extended SMTP 469 client and server may recognize each other, and the server can inform 470 the client as to the service extensions that it supports. 472 Contemporary SMTP implementations MUST support the basic extension 473 mechanisms. For instance, servers MUST support the EHLO command even 474 if they do not implement any specific extensions and clients SHOULD 475 preferentially utilize EHLO rather than HELO. (However, for 476 compatibility with older conforming implementations, SMTP clients and 477 servers MUST support the original HELO mechanisms as a fallback.) 478 Unless the different characteristics of HELO must be identified for 479 interoperability purposes, this document discusses only EHLO. 481 SMTP is widely deployed and high-quality implementations have proven 482 to be very robust. However, the Internet community now considers 483 some services to be important that were not anticipated when the 484 protocol was first designed. If support for those services is to be 485 added, it must be done in a way that permits older implementations to 486 continue working acceptably. The extension framework consists of: 488 o The SMTP command EHLO, superseding the earlier HELO, 490 o a registry of SMTP service extensions, 492 o additional parameters to the SMTP MAIL and RCPT commands, and 494 o optional replacements for commands defined in this protocol, such 495 as for DATA in non-ASCII transmissions (RFC 3030 [32]). 497 SMTP's strength comes primarily from its simplicity. Experience with 498 many protocols has shown that protocols with few options tend towards 499 ubiquity, whereas protocols with many options tend towards obscurity. 501 Each and every extension, regardless of its benefits, must be 502 carefully scrutinized with respect to its implementation, deployment, 503 and interoperability costs. In many cases, the cost of extending the 504 SMTP service will likely outweigh the benefit. 506 2.2.2. Definition and Registration of Extensions 508 The IANA maintains a registry of SMTP service extensions. A 509 corresponding EHLO keyword value is associated with each extension. 510 Each service extension registered with the IANA must be defined in a 511 formal Standards-Track or IESG-approved Experimental protocol 512 document. The definition must include: 514 o the textual name of the SMTP service extension; 516 o the EHLO keyword value associated with the extension; 518 o the syntax and possible values of parameters associated with the 519 EHLO keyword value; 521 o any additional SMTP verbs associated with the extension 522 (additional verbs will usually be, but are not required to be, the 523 same as the EHLO keyword value); 525 o any new parameters the extension associates with the MAIL or RCPT 526 verbs; 528 o a description of how support for the extension affects the 529 behavior of a server and client SMTP; and 531 o the increment by which the extension is increasing the maximum 532 length of the commands MAIL and/or RCPT, over that specified in 533 this Standard. 535 In addition, any EHLO keyword value starting with an upper or lower 536 case "X" refers to a local SMTP service extension used exclusively 537 through bilateral agreement. Keywords beginning with "X" MUST NOT be 538 used in a registered service extension. Conversely, keyword values 539 presented in the EHLO response that do not begin with "X" MUST 540 correspond to a Standard, Standards-Track, or IESG-approved 541 Experimental SMTP service extension registered with IANA. A 542 conforming server MUST NOT offer non-"X"-prefixed keyword values that 543 are not described in a registered extension. 545 Additional verbs and parameter names are bound by the same rules as 546 EHLO keywords; specifically, verbs beginning with "X" are local 547 extensions that may not be registered or standardized. Conversely, 548 verbs not beginning with "X" must always be registered. 550 2.2.3. Special Issues with Extensions 552 Extensions that change fairly basic properties of SMTP operation are 553 permitted. The text in other sections of this document must be 554 understood in that context. In particular, extensions can change the 555 minimum limits specified in Section 4.5.3, can change the ASCII 556 character set requirement as mentioned above, or can introduce some 557 optional modes of message handling. 559 In particular, if an extension implies that the delivery path 560 normally supports special features of that extension, and an 561 intermediate SMTP system finds a next hop that does not support the 562 required extension, it MAY choose, based on the specific extension 563 and circumstances, to requeue the message and try later and/or try an 564 alternate MX host. If this strategy is employed, the timeout to fall 565 back to an unextended format (if one is available) SHOULD be less 566 than the normal timeout for bouncing as undeliverable (e.g., if 567 normal timeout is three days, the requeue timeout before attempting 568 to transmit the mail without the extension might be one day). 570 2.3. SMTP Terminology 571 2.3.1. Mail Objects 573 SMTP transports a mail object. A mail object contains an envelope 574 and content. 576 The SMTP envelope is sent as a series of SMTP protocol units 577 (described in Section 3). It consists of an originator address (to 578 which error reports should be directed), one or more recipient 579 addresses, and optional protocol extension material. Historically, 580 variations on the reverse-path (originator) address specification 581 command (MAIL) could be used to specify alternate delivery modes, 582 such as immediate display; those variations have now been deprecated 583 (see Appendix F and Appendix F.6). 585 The SMTP content is sent in the SMTP DATA protocol unit and has two 586 parts: the header section and the body. If the content conforms to 587 other contemporary standards, the header section consists of a 588 collection of header fields, each consisting of a header name, a 589 colon, and data, structured as in the message format specification 590 (RFC 5322 [11]); the body, if structured, is defined according to 591 MIME (RFC 2045 [24]). The content is textual in nature, expressed 592 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 593 "8BITMIME", RFC 6152 [47]) may relax this restriction for the content 594 body, the content header fields are always encoded using the US-ASCII 595 repertoire. Two MIME extensions (RFC 2047 [25] and RFC 2231 [28]) 596 define an algorithm for representing header values outside the US- 597 ASCII repertoire, while still encoding them using the US-ASCII 598 repertoire. 600 2.3.2. Senders and Receivers 602 In RFC 821, the two hosts participating in an SMTP transaction were 603 described as the "SMTP-sender" and "SMTP-receiver". This document 604 has been changed to reflect current industry terminology and hence 605 refers to them as the "SMTP client" (or sometimes just "the client") 606 and "SMTP server" (or just "the server"), respectively. Since a 607 given host may act both as server and client in a relay situation, 608 "receiver" and "sender" terminology is still used where needed for 609 clarity. 611 2.3.3. Mail Agents and Message Stores 613 Additional mail system terminology became common after RFC 821 was 614 published and, where convenient, is used in this specification. In 615 particular, SMTP servers and clients provide a mail transport service 616 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 617 Agents" (MUAs or UAs) are normally thought of as the sources and 618 targets of mail. At the source, an MUA might collect mail to be 619 transmitted from a user and hand it off to an MTA; the final 620 ("delivery") MTA would be thought of as handing the mail off to an 621 MUA (or at least transferring responsibility to it, e.g., by 622 depositing the message in a "message store"). However, while these 623 terms are used with at least the appearance of great precision in 624 other environments, the implied boundaries between MUAs and MTAs 625 often do not accurately match common, and conforming, practices with 626 Internet mail. Hence, the reader should be cautious about inferring 627 the strong relationships and responsibilities that might be implied 628 if these terms were used elsewhere. 630 2.3.4. Host 632 For the purposes of this specification, a host is a computer system 633 attached to the Internet (or, in some cases, to a private TCP/IP 634 network) and supporting the SMTP protocol. Hosts are known by names 635 (see the next section); they SHOULD NOT be identified by numerical 636 addresses, i.e., by address literals as described in Section 4.1.2. 638 2.3.5. Domain Names 640 A domain name (or often just a "domain") consists of one or more 641 components, separated by dots if more than one appears. In the case 642 of a top-level domain used by itself in an email address, a single 643 string is used without any dots. This makes the requirement, 644 described in more detail below, that only fully-qualified domain 645 names appear in SMTP transactions on the public Internet, 646 particularly important where top-level domains are involved. These 647 components ("labels" in DNS terminology, RFC 1035 [4]) are restricted 648 for SMTP purposes to consist of a sequence of letters, digits, and 649 hyphens drawn from the ASCII character set [2] and conforming to what 650 RFC 1035 Section 2.3.1 calls the "preferred name syntax". Domain 651 names are used as names of hosts and of other entities in the domain 652 name hierarchy. For example, a domain may refer to an alias (label 653 of a CNAME RR) or the label of Mail eXchanger records to be used to 654 deliver mail instead of representing a host name. See RFC 1035 [4] 655 and Section 5 of this specification. 657 The domain name, as described in this document and in RFC 1035 [4], 658 is the entire, fully-qualified name (often referred to as an "FQDN"). 659 A domain name that is not in FQDN form is no more than a local alias. 660 Local aliases MUST NOT appear in any SMTP transaction. 662 Only resolvable, fully-qualified domain names (FQDNs) are permitted 663 when domain names are used in SMTP. 664 [[CREF4: [[5321bis Editor's Note: does "in the public DNS" or 665 equivalent need to be added to "resolvable"???]]]] 666 In other words, names that can be resolved to MX RRs or address 667 (i.e., A or AAAA) RRs (as discussed in Section 5) are permitted, as 668 are CNAME RRs whose targets can be resolved, in turn, to MX or 669 address RRs. 670 [[CREF5: [[5321bis Editor's Note: it is not clear whether "In other 671 words" really meant "for example" or it is was intended that the only 672 labels permitted are those that own records in one of the above RR 673 types]]]] 674 [[CREF6: [[5321bis Editor's Note: More generally, does this section 675 need work to clarify the relationship to private domain names 676 (discussed on SMTP list starting 2013-03-26)]]]] 677 Local nicknames or unqualified names MUST NOT be used. There are two 678 exceptions to the rule requiring FQDNs: 680 o The domain name given in the EHLO command MUST be either a primary 681 host name (a domain name that resolves to an address RR) or, if 682 the host has no name, an address literal, as described in 683 Section 4.1.3 and discussed further in the EHLO discussion of 684 Section 4.1.4. 686 o The reserved mailbox name "postmaster" may be used in a RCPT 687 command without domain qualification (see Section 4.1.1.3) and 688 MUST be accepted if so used. 690 2.3.6. Buffer and State Table 692 SMTP sessions are stateful, with both parties carefully maintaining a 693 common view of the current state. In this document, we model this 694 state by a virtual "buffer" and a "state table" on the server that 695 may be used by the client to, for example, "clear the buffer" or 696 "reset the state table", causing the information in the buffer to be 697 discarded and the state to be returned to some previous state. 699 2.3.7. Commands and Replies 701 SMTP commands and, unless altered by a service extension, message 702 data, are transmitted from the sender to the receiver via the 703 transmission channel in "lines". 705 An SMTP reply is an acknowledgment (positive or negative) sent in 706 "lines" from receiver to sender via the transmission channel in 707 response to a command. The general form of a reply is a numeric 708 completion code (indicating failure or success) usually followed by a 709 text string. The codes are for use by programs and the text is 710 usually intended for human users. RFC 3463 [34], specifies further 711 structuring of the reply strings, including the use of supplemental 712 and more specific completion codes (see also RFC 5248 [46]). 714 2.3.8. Lines 716 Lines consist of zero or more data characters terminated by the 717 sequence ASCII character "CR" (hex value 0D) followed immediately by 718 ASCII character "LF" (hex value 0A). This termination sequence is 719 denoted as in this document. Conforming implementations MUST 720 NOT recognize or generate any other character or character sequence 721 as a line terminator. Limits MAY be imposed on line lengths by 722 servers (see Section 4). 724 In addition, the appearance of "bare" "CR" or "LF" characters in text 725 (i.e., either without the other) has a long history of causing 726 problems in mail implementations and applications that use the mail 727 system as a tool. SMTP client implementations MUST NOT transmit 728 these characters except when they are intended as line terminators 729 and then MUST, as indicated above, transmit them only as a 730 sequence. 732 2.3.9. Message Content and Mail Data 734 The terms "message content" and "mail data" are used interchangeably 735 in this document to describe the material transmitted after the DATA 736 command is accepted and before the end of data indication is 737 transmitted. Message content includes the message header section and 738 the possibly structured message body. The MIME specification (RFC 739 2045 [24]) provides the standard mechanisms for structured message 740 bodies. 742 2.3.10. Originator, Delivery, Relay, and Gateway Systems 744 This specification makes a distinction among four types of SMTP 745 systems, based on the role those systems play in transmitting 746 electronic mail. An "originating" system (sometimes called an SMTP 747 originator) introduces mail into the Internet or, more generally, 748 into a transport service environment. A "delivery" SMTP system is 749 one that receives mail from a transport service environment and 750 passes it to a mail user agent or deposits it in a message store that 751 a mail user agent is expected to subsequently access. A "relay" SMTP 752 system (usually referred to just as a "relay") receives mail from an 753 SMTP client and transmits it, without modification to the message 754 data other than adding trace information, to another SMTP server for 755 further relaying or for delivery. 757 A "gateway" SMTP system (usually referred to just as a "gateway") 758 receives mail from a client system in one transport environment and 759 transmits it to a server system in another transport environment. 760 Differences in protocols or message semantics between the transport 761 environments on either side of a gateway may require that the gateway 762 system perform transformations to the message that are not permitted 763 to SMTP relay systems. For the purposes of this specification, 764 firewalls that rewrite addresses should be considered as gateways, 765 even if SMTP is used on both sides of them (see RFC 2979 [31]). 766 [[CREF7: [5321bis] [[Note in draft/Placeholder: There has been a 767 request to expand this section, possibly into a more extensive model 768 of Internet mail. Comments from others solicited. In particular, 769 does RFC 5598 make that suggestion OBE?]] ]] 771 2.3.11. Mailbox and Address 773 As used in this specification, an "address" is a character string 774 that identifies a user to whom mail will be sent or a location into 775 which mail will be deposited. The term "mailbox" refers to that 776 depository. The two terms are typically used interchangeably unless 777 the distinction between the location in which mail is placed (the 778 mailbox) and a reference to it (the address) is important. An 779 address normally consists of user and domain specifications. The 780 standard mailbox naming convention is defined to be "local- 781 part@domain"; contemporary usage permits a much broader set of 782 applications than simple "user names". Consequently, and due to a 783 long history of problems when intermediate hosts have attempted to 784 optimize transport by modifying them, the local-part MUST be 785 interpreted and assigned semantics only by the host specified in the 786 domain part of the address. 788 2.4. General Syntax Principles and Transaction Model 790 SMTP commands and replies have a rigid syntax. All commands begin 791 with a command verb. All replies begin with a three digit numeric 792 code. In some commands and replies, arguments are required following 793 the verb or reply code. Some commands do not accept arguments (after 794 the verb), and some reply codes are followed, sometimes optionally, 795 by free form text. In both cases, where text appears, it is 796 separated from the verb or reply code by a space character. Complete 797 definitions of commands and replies appear in Section 4. 799 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 800 and extension name keywords) are not case sensitive, with the sole 801 exception in this specification of a mailbox local-part (SMTP 802 Extensions may explicitly specify case-sensitive elements). That is, 803 a command verb, an argument value other than a mailbox local-part, 804 and free form text MAY be encoded in upper case, lower case, or any 805 mixture of upper and lower case with no impact on its meaning. The 806 local-part of a mailbox MUST BE treated as case sensitive. 807 Therefore, SMTP implementations MUST take care to preserve the case 808 of mailbox local-parts. In particular, for some hosts, the user 809 "smith" is different from the user "Smith". However, exploiting the 810 case sensitivity of mailbox local-parts impedes interoperability and 811 is discouraged. Mailbox domains follow normal DNS rules and are 812 hence not case sensitive. 814 A few SMTP servers, in violation of this specification (and RFC 821) 815 require that command verbs be encoded by clients in upper case. 816 Implementations MAY wish to employ this encoding to accommodate those 817 servers. 819 The argument clause consists of a variable-length character string 820 ending with the end of the line, i.e., with the character sequence 821 . The receiver will take no action until this sequence is 822 received. 824 The syntax for each command is shown with the discussion of that 825 command. Common elements and parameters are shown in Section 4.1.2. 827 Commands and replies are composed of characters from the ASCII 828 character set [2]. When the transport service provides an 8-bit byte 829 (octet) transmission channel, each 7-bit character is transmitted, 830 right justified, in an octet with the high-order bit cleared to zero. 831 More specifically, the unextended SMTP service provides 7-bit 832 transport only. An originating SMTP client that has not successfully 833 negotiated an appropriate extension with a particular server (see the 834 next paragraph) MUST NOT transmit messages with information in the 835 high-order bit of octets. If such messages are transmitted in 836 violation of this rule, receiving SMTP servers MAY clear the high- 837 order bit or reject the message as invalid. In general, a relay SMTP 838 SHOULD assume that the message content it has received is valid and, 839 assuming that the envelope permits doing so, relay it without 840 inspecting that content. Of course, if the content is mislabeled and 841 the data path cannot accept the actual content, this may result in 842 the ultimate delivery of a severely garbled message to the recipient. 843 Delivery SMTP systems MAY reject such messages, or return them as 844 undeliverable, rather than deliver them. In the absence of a server- 845 offered extension explicitly permitting it, a sending SMTP system is 846 not permitted to send envelope commands in any character set other 847 than US-ASCII. Receiving systems SHOULD reject such commands, 848 normally using "500 syntax error - invalid character" replies. 850 8-bit message content transmission MAY be requested of the server by 851 a client using extended SMTP facilities, notably the "8BITMIME" 852 extension, RFC 6152 [47]. 8BITMIME SHOULD be supported by SMTP 853 servers. However, it MUST NOT be construed as authorization to 854 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 855 transmission of any envelope material in other than ASCII. 8BITMIME 856 MUST NOT be requested by senders for material with the high bit on 857 that is not in MIME format with an appropriate content-transfer 858 encoding; servers MAY reject such messages. 860 The metalinguistic notation used in this document corresponds to the 861 "Augmented BNF" used in other Internet mail system documents. The 862 reader who is not familiar with that syntax should consult the ABNF 863 specification in RFC 5234 [10]. Metalanguage terms used in running 864 text are surrounded by pointed brackets (e.g., ) for clarity. 865 The reader is cautioned that the grammar expressed in the 866 metalanguage is not comprehensive. There are many instances in which 867 provisions in the text constrain or otherwise modify the syntax or 868 semantics implied by the grammar. 870 3. The SMTP Procedures: An Overview 872 This section contains descriptions of the procedures used in SMTP: 873 session initiation, mail transaction, forwarding mail, verifying 874 mailbox names and expanding mailing lists, and opening and closing 875 exchanges. Comments on relaying, a note on mail domains, and a 876 discussion of changing roles are included at the end of this section. 877 Several complete scenarios are presented in Appendix D. 879 3.1. Session Initiation 881 An SMTP session is initiated when a client opens a connection to a 882 server and the server responds with an opening message. 884 SMTP server implementations MAY include identification of their 885 software and version information in the connection greeting reply 886 after the 220 code, a practice that permits more efficient isolation 887 and repair of any problems. Implementations MAY make provision for 888 SMTP servers to disable the software and version announcement where 889 it causes security concerns. While some systems also identify their 890 contact point for mail problems, this is not a substitute for 891 maintaining the required "postmaster" address (see Section 4). 893 The SMTP protocol allows a server to formally reject a mail session 894 while still allowing the initial connection as follows: a 554 895 response MAY be given in the initial connection opening message 896 instead of the 220. A server taking this approach MUST still wait 897 for the client to send a QUIT (see Section 4.1.1.10) before closing 898 the connection and SHOULD respond to any intervening commands with 899 "503 bad sequence of commands". Since an attempt to make an SMTP 900 connection to such a system is probably in error, a server returning 901 a 554 response on connection opening SHOULD provide enough 902 information in the reply text to facilitate debugging of the sending 903 system. 905 3.2. Client Initiation 907 Once the server has sent the greeting (welcoming) message and the 908 client has received it, the client normally sends the EHLO command to 909 the server, indicating the client's identity. In addition to opening 910 the session, use of EHLO indicates that the client is able to process 911 service extensions and requests that the server provide a list of the 912 extensions it supports. Older SMTP systems that are unable to 913 support service extensions, and contemporary clients that do not 914 require service extensions in the mail session being initiated, MAY 915 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 916 style response to a HELO command. For a particular connection 917 attempt, if the server returns a "command not recognized" response to 918 EHLO, the client SHOULD be able to fall back and send HELO. 920 In the EHLO command, the host sending the command identifies itself; 921 the command may be interpreted as saying "Hello, I am " (and, 922 in the case of EHLO, "and I support service extension requests"). 924 3.3. Mail Transactions 926 There are three steps to SMTP mail transactions. The transaction 927 starts with a MAIL command that gives the sender identification. (In 928 general, the MAIL command may be sent only when no mail transaction 929 is in progress; see Section 4.1.4.) A series of one or more RCPT 930 commands follows, giving the receiver information. Then, a DATA 931 command initiates transfer of the mail data and is terminated by the 932 "end of mail" data indicator, which also confirms the transaction. 934 The first step in the procedure is the MAIL command. 936 MAIL FROM: [SP ] 938 This command tells the SMTP-receiver that a new mail transaction is 939 starting and to reset all its state tables and buffers, including any 940 recipients or mail data. The portion of the first or 941 only argument contains the source mailbox (between "<" and ">" 942 brackets), which can be used to report errors (see Section 4.2 for a 943 discussion of error reporting). If accepted, the SMTP server returns 944 a "250 OK" reply. If the mailbox specification is not acceptable for 945 some reason, the server MUST return a reply indicating whether the 946 failure is permanent (i.e., will occur again if the client tries to 947 send the same address again) or temporary (i.e., the address might be 948 accepted if the client tries again later). Despite the apparent 949 scope of this requirement, there are circumstances in which the 950 acceptability of the reverse-path may not be determined until one or 951 more forward-paths (in RCPT commands) can be examined. In those 952 cases, the server MAY reasonably accept the reverse-path (with a 250 953 reply) and then report problems after the forward-paths are received 954 and examined. Normally, failures produce 550 or 553 replies. 956 Historically, the was permitted to contain more than 957 just a mailbox; however, contemporary systems SHOULD NOT use source 958 routing (see Appendix C). 960 The optional are associated with negotiated SMTP 961 service extensions (see Section 2.2). 963 The second step in the procedure is the RCPT command. This step of 964 the procedure can be repeated any number of times. 966 RCPT TO: [ SP ] 968 The first or only argument to this command includes a forward-path 969 (normally a mailbox and domain, always surrounded by "<" and ">" 970 brackets) identifying one recipient. If accepted, the SMTP server 971 returns a "250 OK" reply and stores the forward-path. If the 972 recipient is known not to be a deliverable address, the SMTP server 973 returns a 550 reply, typically with a string such as "no such user - 974 " and the mailbox name (other circumstances and reply codes are 975 possible). 977 The can contain more than just a mailbox. 978 Historically, the was permitted to contain a source 979 routing list of hosts and the destination mailbox; however, 980 contemporary SMTP clients SHOULD NOT utilize source routes (see 981 Appendix C). Servers MUST be prepared to encounter a list of source 982 routes in the forward-path, but they SHOULD ignore the routes or MAY 983 decline to support the relaying they imply. Similarly, servers MAY 984 decline to accept mail that is destined for other hosts or systems. 985 These restrictions make a server useless as a relay for clients that 986 do not support full SMTP functionality. Consequently, restricted- 987 capability clients MUST NOT assume that any SMTP server on the 988 Internet can be used as their mail processing (relaying) site. If a 989 RCPT command appears without a previous MAIL command, the server MUST 990 return a 503 "Bad sequence of commands" response. The optional 991 are associated with negotiated SMTP service 992 extensions (see Section 2.2). [[CREF8: [5321bis] JcK Note for 993 2821ter (5321bis): this section would be improved by being more 994 specific about where mail transactions begin and end and then talking 995 about "transaction state" here, rather than specific prior commands. 996 --JcK]] 998 Since it has been a common source of errors, it is worth noting that 999 spaces are not permitted on either side of the colon following FROM 1000 in the MAIL command or TO in the RCPT command. The syntax is exactly 1001 as given above. 1003 The third step in the procedure is the DATA command (or some 1004 alternative specified in a service extension). 1006 DATA 1008 If accepted, the SMTP server returns a 354 Intermediate reply and 1009 considers all succeeding lines up to but not including the end of 1010 mail data indicator to be the message text. When the end of text is 1011 successfully received and stored, the SMTP-receiver sends a "250 OK" 1012 reply. 1014 Since the mail data is sent on the transmission channel, the end of 1015 mail data must be indicated so that the command and reply dialog can 1016 be resumed. SMTP indicates the end of the mail data by sending a 1017 line containing only a "." (period or full stop). A transparency 1018 procedure is used to prevent this from interfering with the user's 1019 text (see Section 4.5.2). 1021 The end of mail data indicator also confirms the mail transaction and 1022 tells the SMTP server to now process the stored recipients and mail 1023 data. If accepted, the SMTP server returns a "250 OK" reply. The 1024 DATA command can fail at only two points in the protocol exchange: 1026 If there was no MAIL, or no RCPT, command, or all such commands were 1027 rejected, the server MAY return a "command out of sequence" (503) or 1028 "no valid recipients" (554) reply in response to the DATA command. 1029 If one of those replies (or any other 5yz reply) is received, the 1030 client MUST NOT send the message data; more generally, message data 1031 MUST NOT be sent unless a 354 reply is received. 1033 If the verb is initially accepted and the 354 reply issued, the DATA 1034 command should fail only if the mail transaction was incomplete (for 1035 example, no recipients), if resources were unavailable (including, of 1036 course, the server unexpectedly becoming unavailable), or if the 1037 server determines that the message should be rejected for policy or 1038 other reasons. 1040 However, in practice, some servers do not perform recipient 1041 verification until after the message text is received. These servers 1042 SHOULD treat a failure for one or more recipients as a "subsequent 1043 failure" and return a mail message as discussed in Section 6 and, in 1044 particular, in Section 6.1. Using a "550 mailbox not found" (or 1045 equivalent) reply code after the data are accepted makes it difficult 1046 or impossible for the client to determine which recipients failed. 1048 When the RFC 822 format ([12], [11]) is being used, the mail data 1049 include the header fields such as those named Date, Subject, To, Cc, 1050 and From. Server SMTP systems SHOULD NOT reject messages based on 1051 perceived defects in the RFC 822 or MIME (RFC 2045 [24]) message 1052 header section or message body. In particular, they MUST NOT reject 1053 messages in which the numbers of Resent-header fields do not match or 1054 Resent-to appears without Resent-from and/or Resent-date. 1056 Mail transaction commands MUST be used in the order discussed above. 1058 3.4. Forwarding for Address Correction or Updating 1060 Forwarding support is most often required to consolidate and simplify 1061 addresses within, or relative to, some enterprise and less frequently 1062 to establish addresses to link a person's prior address with a 1063 current one. Silent forwarding of messages (without server 1064 notification to the sender), for security or non-disclosure purposes, 1065 is common in the contemporary Internet. 1067 In both the enterprise and the "new address" cases, information 1068 hiding (and sometimes security) considerations argue against exposure 1069 of the "final" address through the SMTP protocol as a side effect of 1070 the forwarding activity. This may be especially important when the 1071 final address may not even be reachable by the sender. Consequently, 1072 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1073 especially the 251 (corrected destination) and 551 reply codes from 1074 RCPT must be evaluated carefully by implementers and, when they are 1075 available, by those configuring systems (see also Section 7.4). 1077 In particular: 1079 o Servers MAY forward messages when they are aware of an address 1080 change. When they do so, they MAY either provide address-updating 1081 information with a 251 code, or may forward "silently" and return 1082 a 250 code. However, if a 251 code is used, they MUST NOT assume 1083 that the client will actually update address information or even 1084 return that information to the user. 1086 Alternately, 1088 o Servers MAY reject messages or return them as non-deliverable when 1089 they cannot be delivered precisely as addressed. When they do so, 1090 they MAY either provide address-updating information with a 551 1091 code, or may reject the message as undeliverable with a 550 code 1092 and no address-specific information. However, if a 551 code is 1093 used, they MUST NOT assume that the client will actually update 1094 address information or even return that information to the user. 1096 SMTP server implementations that support the 251 and/or 551 reply 1097 codes SHOULD provide configuration mechanisms so that sites that 1098 conclude that they would undesirably disclose information can disable 1099 or restrict their use. 1101 3.5. Commands for Debugging Addresses 1103 3.5.1. Overview 1105 SMTP provides commands to verify a user name or obtain the content of 1106 a mailing list. This is done with the VRFY and EXPN commands, which 1107 have character string arguments. Implementations SHOULD support VRFY 1108 and EXPN (however, see Section 3.5.2 and Section 7.3). 1110 For the VRFY command, the string is a user name or a user name and 1111 domain (see below). If a normal (i.e., 250) response is returned, 1112 the response MAY include the full name of the user and MUST include 1113 the mailbox of the user. It MUST be in either of the following 1114 forms: 1116 User Name 1117 local-part@domain 1119 When a name that is the argument to VRFY could identify more than one 1120 mailbox, the server MAY either note the ambiguity or identify the 1121 alternatives. In other words, any of the following are legitimate 1122 responses to VRFY: 1124 553 User ambiguous 1126 or 1128 553- Ambiguous; Possibilities are 1129 553-Joe Smith 1130 553-Harry Smith 1131 553 Melvin Smith 1133 or 1135 553-Ambiguous; Possibilities 1136 553- 1137 553- 1138 553 1140 Under normal circumstances, a client receiving a 553 reply would be 1141 expected to expose the result to the user. Use of exactly the forms 1142 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1143 supplemented by extended reply codes, such as those described in RFC 1144 3463 [34], will facilitate automated translation into other languages 1145 as needed. Of course, a client that was highly automated or that was 1146 operating in another language than English might choose to try to 1147 translate the response to return some other indication to the user 1148 than the literal text of the reply, or to take some automated action 1149 such as consulting a directory service for additional information 1150 before reporting to the user. 1152 For the EXPN command, the string identifies a mailing list, and the 1153 successful (i.e., 250) multiline response MAY include the full name 1154 of the users and MUST give the mailboxes on the mailing list. 1156 In some hosts, the distinction between a mailing list and an alias 1157 for a single mailbox is a bit fuzzy, since a common data structure 1158 may hold both types of entries, and it is possible to have mailing 1159 lists containing only one mailbox. If a request is made to apply 1160 VRFY to a mailing list, a positive response MAY be given if a message 1161 so addressed would be delivered to everyone on the list, otherwise an 1162 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1163 user" or "252 Unable to verify members of mailing list"). If a 1164 request is made to expand a user name, the server MAY return a 1165 positive response consisting of a list containing one name, or an 1166 error MAY be reported (e.g., "550 That is a user name, not a mailing 1167 list"). 1169 In the case of a successful multiline reply (normal for EXPN), 1170 exactly one mailbox is to be specified on each line of the reply. 1171 The case of an ambiguous request is discussed above. 1173 "User name" is a fuzzy term and has been used deliberately. An 1174 implementation of the VRFY or EXPN commands MUST include at least 1175 recognition of local mailboxes as "user names". However, since 1176 current Internet practice often results in a single host handling 1177 mail for multiple domains, hosts, especially hosts that provide this 1178 functionality, SHOULD accept the "local-part@domain" form as a "user 1179 name"; hosts MAY also choose to recognize other strings as "user 1180 names". 1182 The case of expanding a mailbox list requires a multiline reply, such 1183 as: 1185 C: EXPN Example-People 1186 S: 250-Jon Postel 1187 S: 250-Fred Fonebone 1188 S: 250 Sam Q. Smith 1190 or 1191 C: EXPN Executive-Washroom-List 1192 S: 550 Access Denied to You. 1194 The character string arguments of the VRFY and EXPN commands cannot 1195 be further restricted due to the variety of implementations of the 1196 user name and mailbox list concepts. On some systems, it may be 1197 appropriate for the argument of the EXPN command to be a file name 1198 for a file containing a mailing list, but again there are a variety 1199 of file naming conventions in the Internet. Similarly, historical 1200 variations in what is returned by these commands are such that the 1201 response SHOULD be interpreted very carefully, if at all, and SHOULD 1202 generally only be used for diagnostic purposes. 1204 3.5.2. VRFY Normal Response 1206 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1207 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1209 domain name. In circumstances exceptional enough to justify 1210 violating the intent of this specification, free-form text MAY be 1211 returned. In order to facilitate parsing by both computers and 1212 people, addresses SHOULD appear in pointed brackets. When addresses, 1213 rather than free-form debugging information, are returned, EXPN and 1214 VRFY MUST return only valid domain addresses that are usable in SMTP 1215 RCPT commands. Consequently, if an address implies delivery to a 1216 program or other system, the mailbox name used to reach that target 1217 MUST be given. Paths (explicit source routes) MUST NOT be returned 1218 by VRFY or EXPN. 1220 Server implementations SHOULD support both VRFY and EXPN. For 1221 security reasons, implementations MAY provide local installations a 1222 way to disable either or both of these commands through configuration 1223 options or the equivalent (see Section 7.3). When these commands are 1224 supported, they are not required to work across relays when relaying 1225 is supported. Since they were both optional in RFC 821, but VRFY was 1226 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1227 listed as a service extension in an EHLO response. VRFY MAY be 1228 listed as a convenience but, since support for it is required, SMTP 1229 clients are not required to check for its presence on the extension 1230 list before using it. 1232 3.5.3. Meaning of VRFY or EXPN Success Response 1234 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1235 command unless it has actually verified the address. In particular, 1236 a server MUST NOT return 250 if all it has done is to verify that the 1237 syntax given is valid. In that case, 502 (Command not implemented) 1238 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1239 stated elsewhere, implementation (in the sense of actually validating 1240 addresses and returning information) of VRFY and EXPN are strongly 1241 recommended. Hence, implementations that return 500 or 502 for VRFY 1242 are not in full compliance with this specification. 1244 There may be circumstances where an address appears to be valid but 1245 cannot reasonably be verified in real time, particularly when a 1246 server is acting as a mail exchanger for another server or domain. 1247 "Apparent validity", in this case, would normally involve at least 1248 syntax checking and might involve verification that any domains 1249 specified were ones to which the host expected to be able to relay 1250 mail. In these situations, reply code 252 SHOULD be returned. These 1251 cases parallel the discussion of RCPT verification in Section 2.1. 1252 Similarly, the discussion in Section 3.4 applies to the use of reply 1253 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are 1254 recognized but that would be forwarded or rejected were mail received 1255 for them. Implementations generally SHOULD be more aggressive about 1256 address verification in the case of VRFY than in the case of RCPT, 1257 even if it takes a little longer to do so. 1259 3.5.4. Semantics and Applications of EXPN 1261 EXPN is often very useful in debugging and understanding problems 1262 with mailing lists and multiple-target-address aliases. Some systems 1263 have attempted to use source expansion of mailing lists as a means of 1264 eliminating duplicates. The propagation of aliasing systems with 1265 mail on the Internet for hosts (typically with MX and CNAME DNS 1266 records), for mailboxes (various types of local host aliases), and in 1267 various proxying arrangements has made it nearly impossible for these 1268 strategies to work consistently, and mail systems SHOULD NOT attempt 1269 them. 1271 3.6. Relaying and Mail Routing 1273 3.6.1. Source Routes and Relaying 1275 In general, the availability of Mail eXchanger records in the domain 1276 name system (RFC 1035 [4], RFC 974 [15]) makes the use of explicit 1277 source routes in the Internet mail system unnecessary. Many 1278 historical problems with the interpretation of explicit source routes 1279 have made their use undesirable. SMTP clients SHOULD NOT generate 1280 explicit source routes except under unusual circumstances. SMTP 1281 servers MAY decline to act as mail relays or to accept addresses that 1282 specify source routes. When route information is encountered, SMTP 1283 servers MAY ignore the route information and simply send to the final 1284 destination specified as the last element in the route and SHOULD do 1285 so. There has been an invalid practice of using names that do not 1286 appear in the DNS as destination names, with the senders counting on 1287 the intermediate hosts specified in source routing to resolve any 1288 problems. If source routes are stripped, this practice will cause 1289 failures. This is one of several reasons why SMTP clients MUST NOT 1290 generate invalid source routes or depend on serial resolution of 1291 names in such routes. [[CREF9: [5321bis] Jck 20091023: "of names..." 1292 added for clarity"]] 1294 When source routes are not used, the process described in RFC 821 for 1295 constructing a reverse-path from the forward-path is not applicable 1296 and the reverse-path at the time of delivery will simply be the 1297 address that appeared in the MAIL command. 1299 3.6.2. Mail eXchange Records and Relaying 1301 A relay SMTP server is usually the target of a DNS MX record that 1302 designates it, rather than the final delivery system. The relay 1303 server may accept or reject the task of relaying the mail in the same 1304 way it accepts or rejects mail for a local user. If it accepts the 1305 task, it then becomes an SMTP client, establishes a transmission 1306 channel to the next SMTP server specified in the DNS (according to 1307 the rules in Section 5), and sends it the mail. If it declines to 1308 relay mail to a particular address for policy reasons, a 550 response 1309 SHOULD be returned. 1311 This specification does not deal with the verification of return 1312 paths for use in delivery notifications. Recent work, such as that 1313 on SPF [41] and DKIM [43] [44], has been done to provide ways to 1314 ascertain that an address is valid or belongs to the person who 1315 actually sent the message. 1316 [[5321bis Editor's Note: Proposed erratum (4055) suggests that DKIM 1317 and SPF have nothing to do with this and that everything after the 1318 first sentence should be dropped. An alternative would be to tune 1319 the texts. ???]] 1320 A server MAY attempt to verify the return path before using its 1321 address for delivery notifications, but methods of doing so are not 1322 defined here nor is any particular method recommended at this time. 1324 3.6.3. Message Submission Servers as Relays 1326 Many mail-sending clients exist, especially in conjunction with 1327 facilities that receive mail via POP3 or IMAP, that have limited 1328 capability to support some of the requirements of this specification, 1329 such as the ability to queue messages for subsequent delivery 1330 attempts. For these clients, it is common practice to make private 1331 arrangements to send all messages to a single server for processing 1332 and subsequent distribution. SMTP, as specified here, is not ideally 1333 suited for this role. A standardized mail submission protocol has 1334 been developed that is gradually superseding practices based on SMTP 1335 (see RFC 4409 [42]). In any event, because these arrangements are 1336 private and fall outside the scope of this specification, they are 1337 not described here. 1339 It is important to note that MX records can point to SMTP servers 1340 that act as gateways into other environments, not just SMTP relays 1341 and final delivery systems; see Sections 3.7 and 5. 1343 If an SMTP server has accepted the task of relaying the mail and 1344 later finds that the destination is incorrect or that the mail cannot 1345 be delivered for some other reason, then it MUST construct an 1346 "undeliverable mail" notification message and send it to the 1347 originator of the undeliverable mail (as indicated by the reverse- 1348 path). Formats specified for non-delivery reports by other standards 1349 (see, for example, RFC 3461 [33] and RFC 3464 [35]) SHOULD be used if 1350 possible. 1352 This notification message must be from the SMTP server at the relay 1353 host or the host that first determines that delivery cannot be 1354 accomplished. Of course, SMTP servers MUST NOT send notification 1355 messages about problems transporting notification messages. One way 1356 to prevent loops in error reporting is to specify a null reverse-path 1357 in the MAIL command of a notification message. When such a message 1358 is transmitted, the reverse-path MUST be set to null (see 1359 Section 4.5.5 for additional discussion). A MAIL command with a null 1360 reverse-path appears as follows: 1362 MAIL FROM:<> 1364 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1365 act upon the header section or body of the message data and MUST NOT 1366 do so except to add its own "Received:" header field (Section 4.4) 1367 and, optionally, to attempt to detect looping in the mail system (see 1368 Section 6.3). Of course, this prohibition also applies to any 1369 modifications of these header fields or text (see also Section 7.9). 1371 3.7. Mail Gatewaying 1373 While the relay function discussed above operates within the Internet 1374 SMTP transport service environment, MX records or various forms of 1375 explicit routing may require that an intermediate SMTP server perform 1376 a translation function between one transport service and another. As 1377 discussed in Section 2.3.10, when such a system is at the boundary 1378 between two transport service environments, we refer to it as a 1379 "gateway" or "gateway SMTP". 1381 Gatewaying mail between different mail environments, such as 1382 different mail formats and protocols, is complex and does not easily 1383 yield to standardization. However, some general requirements may be 1384 given for a gateway between the Internet and another mail 1385 environment. 1387 3.7.1. Header Fields in Gatewaying 1389 Header fields MAY be rewritten when necessary as messages are 1390 gatewayed across mail environment boundaries. This may involve 1391 inspecting the message body or interpreting the local-part of the 1392 destination address in spite of the prohibitions in Section 6.4. 1394 Other mail systems gatewayed to the Internet often use a subset of 1395 the RFC 822 header section or provide similar functionality with a 1396 different syntax, but some of these mail systems do not have an 1397 equivalent to the SMTP envelope. Therefore, when a message leaves 1398 the Internet environment, it may be necessary to fold the SMTP 1399 envelope information into the message header section. A possible 1400 solution would be to create new header fields to carry the envelope 1401 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1402 would require changes in mail programs in foreign environments and 1403 might risk disclosure of private information (see Section 7.2). 1405 3.7.2. Received Lines in Gatewaying 1407 When forwarding a message into or out of the Internet environment, a 1408 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1409 way a Received: line that is already in the header section. 1411 "Received:" header fields of messages originating from other 1412 environments may not conform exactly to this specification. However, 1413 the most important use of Received: lines is for debugging mail 1414 faults, and this debugging can be severely hampered by well-meaning 1415 gateways that try to "fix" a Received: line. As another consequence 1416 of trace header fields arising in non-SMTP environments, receiving 1417 systems MUST NOT reject mail based on the format of a trace header 1418 field and SHOULD be extremely robust in the light of unexpected 1419 information or formats in those header fields. 1421 The gateway SHOULD indicate the environment and protocol in the "via" 1422 clauses of Received header field(s) that it supplies. 1424 3.7.3. Addresses in Gatewaying 1426 From the Internet side, the gateway SHOULD accept all valid address 1427 formats in SMTP commands and in the RFC 822 header section, and all 1428 valid RFC 822 messages. Addresses and header fields generated by 1429 gateways MUST conform to applicable standards (including this one and 1430 RFC 5322 [11]). Gateways are, of course, subject to the same rules 1431 for handling source routes as those described for other SMTP systems 1432 in Section 3.3. 1434 3.7.4. Other Header Fields in Gatewaying 1436 The gateway MUST ensure that all header fields of a message that it 1437 forwards into the Internet mail environment meet the requirements for 1438 Internet mail. In particular, all addresses in "From:", "To:", 1439 "Cc:", etc., header fields MUST be transformed (if necessary) to 1440 satisfy the standard header syntax of RFC 5322 [11], MUST reference 1441 only fully-qualified domain names, and MUST be effective and useful 1442 for sending replies. The translation algorithm used to convert mail 1443 from the Internet protocols to another environment's protocol SHOULD 1444 ensure that error messages from the foreign mail environment are 1445 delivered to the reverse-path from the SMTP envelope, not to an 1446 address in the "From:", "Sender:", or similar header fields of the 1447 message. 1449 3.7.5. Envelopes in Gatewaying 1451 Similarly, when forwarding a message from another environment into 1452 the Internet, the gateway SHOULD set the envelope return path in 1453 accordance with an error message return address, if supplied by the 1454 foreign environment. If the foreign environment has no equivalent 1455 concept, the gateway must select and use a best approximation, with 1456 the message originator's address as the default of last resort. 1458 3.8. Terminating Sessions and Connections 1460 An SMTP connection is terminated when the client sends a QUIT 1461 command. The server responds with a positive reply code, after which 1462 it closes the connection. 1464 An SMTP server MUST NOT intentionally close the connection under 1465 normal operational circumstances (see Section 7.8) except: 1467 o After receiving a QUIT command and responding with a 221 reply. 1469 o After detecting the need to shut down the SMTP service and 1470 returning a 421 reply code. This reply code can be issued after 1471 the server receives any command or, if necessary, asynchronously 1472 from command receipt (on the assumption that the client will 1473 receive it after the next command is issued). 1475 o After a timeout, as specified in Section 4.5.3.2, occurs waiting 1476 for the client to send a command or data. 1478 In particular, a server that closes connections in response to 1479 commands that are not understood is in violation of this 1480 specification. Servers are expected to be tolerant of unknown 1481 commands, issuing a 500 reply and awaiting further instructions from 1482 the client. 1484 An SMTP server that is forcibly shut down via external means SHOULD 1485 attempt to send a line containing a 421 reply code to the SMTP client 1486 before exiting. The SMTP client will normally read the 421 reply 1487 code after sending its next command. 1489 SMTP clients that experience a connection close, reset, or other 1490 communications failure due to circumstances not under their control 1491 (in violation of the intent of this specification but sometimes 1492 unavoidable) SHOULD, to maintain the robustness of the mail system, 1493 treat the mail transaction as if a 421 response had been received and 1494 act accordingly. 1496 3.9. Mailing Lists and Aliases 1498 [[CREF10: [5321bis] If "alias and list models" are explained 1499 elsewhere, cross reference". Also note that this section appears to 1500 prohibit an exploder from adding List-* headers. That needs to be 1501 finessed.]] 1502 An SMTP-capable host SHOULD support both the alias and the list 1503 models of address expansion for multiple delivery. When a message is 1504 delivered or forwarded to each address of an expanded list form, the 1505 return address in the envelope ("MAIL FROM:") MUST be changed to be 1506 the address of a person or other entity who administers the list. 1507 However, in this case, the message header section (RFC 5322 [11]) 1508 MUST be left unchanged; in particular, the "From" field of the header 1509 section is unaffected. 1511 An important mail facility is a mechanism for multi-destination 1512 delivery of a single message, by transforming (or "expanding" or 1513 "exploding") a pseudo-mailbox address into a list of destination 1514 mailbox addresses. When a message is sent to such a pseudo-mailbox 1515 (sometimes called an "exploder"), copies are forwarded or 1516 redistributed to each mailbox in the expanded list. Servers SHOULD 1517 simply utilize the addresses on the list; application of heuristics 1518 or other matching rules to eliminate some addresses, such as that of 1519 the originator, is strongly discouraged. We classify such a pseudo- 1520 mailbox as an "alias" or a "list", depending upon the expansion 1521 rules. 1523 3.9.1. Alias 1525 To expand an alias, the recipient mailer simply replaces the pseudo- 1526 mailbox address in the envelope with each of the expanded addresses 1527 in turn; the rest of the envelope and the message body are left 1528 unchanged. The message is then delivered or forwarded to each 1529 expanded address. 1531 3.9.2. List 1533 A mailing list may be said to operate by "redistribution" rather than 1534 by "forwarding". To expand a list, the recipient mailer replaces the 1535 pseudo-mailbox address in the envelope with each of the expanded 1536 addresses in turn. The return (backward-pointing) address in the 1537 envelope is changed so that all error messages generated by the final 1538 deliveries will be returned to a list administrator, not to the 1539 message originator, who generally has no control over the contents of 1540 the list and will typically find error messages annoying. Note that 1541 the key difference between handling aliases (Section 3.9.1) and 1542 forwarding (this subsection) is the change to the backward-pointing 1543 address in this case. When a list constrains its processing to the 1544 very limited set of modifications and actions described here, it is 1545 attempting to emulate an MTA; such lists can be treated as a 1546 continuation in email transit. 1548 There exist mailing lists that perform additional, sometimes 1549 extensive, modifications to a message and its envelope. Such mailing 1550 lists need to be viewed as full MUAs, which accept a delivery and 1551 post a new message. 1553 4. The SMTP Specifications 1555 4.1. SMTP Commands 1557 4.1.1. Command Semantics and Syntax 1559 The SMTP commands define the mail transfer or the mail system 1560 function requested by the user. SMTP commands are character strings 1561 terminated by . The commands themselves are alphabetic 1562 characters terminated by if parameters follow and 1563 otherwise. (In the interest of improved interoperability, SMTP 1564 receivers SHOULD tolerate trailing white space before the terminating 1565 .) The syntax of the local part of a mailbox MUST conform to 1566 receiver site conventions and the syntax specified in Section 4.1.2. 1567 The SMTP commands are discussed below. The SMTP replies are 1568 discussed in Section 4.2. 1570 A mail transaction involves several data objects that are 1571 communicated as arguments to different commands. The reverse-path is 1572 the argument of the MAIL command, the forward-path is the argument of 1573 the RCPT command, and the mail data is the argument of the DATA 1574 command. These arguments or data objects must be transmitted and 1575 held, pending the confirmation communicated by the end of mail data 1576 indication that finalizes the transaction. The model for this is 1577 that distinct buffers are provided to hold the types of data objects; 1578 that is, there is a reverse-path buffer, a forward-path buffer, and a 1579 mail data buffer. Specific commands cause information to be appended 1580 to a specific buffer, or cause one or more buffers to be cleared. 1582 Several commands (RSET, DATA, QUIT) are specified as not permitting 1583 parameters. In the absence of specific extensions offered by the 1584 server and accepted by the client, clients MUST NOT send such 1585 parameters and servers SHOULD reject commands containing them as 1586 having invalid syntax. 1588 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1590 These commands are used to identify the SMTP client to the SMTP 1591 server. The argument clause contains the fully-qualified domain name 1592 of the SMTP client, if one is available. In situations in which the 1593 SMTP client system does not have a meaningful domain name (e.g., when 1594 its address is dynamically allocated and no reverse mapping record is 1595 available), the client SHOULD send an address literal (see 1596 Section 4.1.3). 1598 RFC 2821, and some earlier informal practices, encouraged following 1599 the literal by information that would help to identify the client 1600 system. That convention was not widely supported, and many SMTP 1601 servers considered it an error. In the interest of interoperability, 1602 it is probably wise for servers to be prepared for this string to 1603 occur, but SMTP clients SHOULD NOT send it. 1605 The SMTP server identifies itself to the SMTP client in the 1606 connection greeting reply and in the response to this command. 1608 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1609 command. If the SMTP server supports the SMTP service extensions, it 1610 will give a successful response, a failure response, or an error 1611 response. If the SMTP server, in violation of this specification, 1612 does not support any SMTP service extensions, it will generate an 1613 error response. Older client SMTP systems MAY, as discussed above, 1614 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1615 support the HELO command and reply properly to it. In any event, a 1616 client MUST issue HELO or EHLO before starting a mail transaction. 1618 These commands, and a "250 OK" reply to one of them, confirm that 1619 both the SMTP client and the SMTP server are in the initial state, 1620 that is, there is no transaction in progress and all state tables and 1621 buffers are cleared. 1623 Syntax: 1625 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1627 helo = "HELO" SP Domain CRLF 1629 Normally, the response to EHLO will be a multiline reply. Each line 1630 of the response contains a keyword and, optionally, one or more 1631 parameters. Following the normal syntax for multiline replies, these 1632 keywords follow the code (250) and a hyphen for all but the last 1633 line, and the code and a space for the last line. The syntax for a 1634 positive response, using the ABNF notation and terminal symbols of 1635 RFC 5234 [10], is: 1637 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1638 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1639 *( "250-" ehlo-line CRLF ) 1640 "250" SP ehlo-line CRLF ) 1642 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1643 ; string of any characters other than CR or LF 1645 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1647 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1648 ; additional syntax of ehlo-params depends on 1649 ; ehlo-keyword 1651 ehlo-param = 1*(%d33-126) 1652 ; any CHAR excluding and all 1653 ; control characters (US-ASCII 0-31 and 127 1654 ; inclusive) 1656 Although EHLO keywords may be specified in upper, lower, or mixed 1657 case, they MUST always be recognized and processed in a case- 1658 insensitive manner. This is simply an extension of practices 1659 specified in RFC 821 and Section 2.4. 1661 The EHLO response MUST contain keywords (and associated parameters if 1662 required) for all commands not listed as "required" in Section 4.5.1 1663 excepting only private-use commands as described in Section 4.1.5. 1664 Private-use commands MAY be listed. 1666 4.1.1.2. MAIL (MAIL) 1668 This command is used to initiate a mail transaction in which the mail 1669 data is delivered to an SMTP server that may, in turn, deliver it to 1670 one or more mailboxes or pass it on to another system (possibly using 1671 SMTP). The argument clause contains a reverse-path and may contain 1672 optional parameters. In general, the MAIL command may be sent only 1673 when no mail transaction is in progress, see Section 4.1.4. 1675 The reverse-path consists of the sender mailbox. Historically, that 1676 mailbox might optionally have been preceded by a list of hosts, but 1677 that behavior is now deprecated (see Appendix C). In some types of 1678 reporting messages for which a reply is likely to cause a mail loop 1679 (for example, mail delivery and non-delivery notifications), the 1680 reverse-path may be null (see Section 3.6). 1682 This command clears the reverse-path buffer, the forward-path buffer, 1683 and the mail data buffer, and it inserts the reverse-path information 1684 from its argument clause into the reverse-path buffer. 1686 If service extensions were negotiated, the MAIL command may also 1687 carry parameters associated with a particular service extension. 1689 Syntax: 1691 mail = "MAIL FROM:" Reverse-path 1692 [SP Mail-parameters] CRLF 1694 4.1.1.3. RECIPIENT (RCPT) 1696 This command is used to identify an individual recipient of the mail 1697 data; multiple recipients are specified by multiple uses of this 1698 command. The argument clause contains a forward-path and may contain 1699 optional parameters. 1701 The forward-path normally consists of the required destination 1702 mailbox. Sending systems SHOULD NOT generate the optional list of 1703 hosts known as a source route. Receiving systems MUST recognize 1704 source route syntax but SHOULD strip off the source route 1705 specification and utilize the domain name associated with the mailbox 1706 as if the source route had not been provided. 1708 Similarly, relay hosts SHOULD strip or ignore source routes, and 1709 names MUST NOT be copied into the reverse-path. When mail reaches 1710 its ultimate destination (the forward-path contains only a 1711 destination mailbox), the SMTP server inserts it into the destination 1712 mailbox in accordance with its host mail conventions. 1714 This command appends its forward-path argument to the forward-path 1715 buffer; it does not change the reverse-path buffer nor the mail data 1716 buffer. 1718 For example, mail received at relay host xyz.com with envelope 1719 commands 1721 MAIL FROM: 1722 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1724 will normally be sent directly on to host d.bar.org with envelope 1725 commands 1727 MAIL FROM: 1728 RCPT TO: 1730 As provided in Appendix C, xyz.com MAY also choose to relay the 1731 message to hosta.int, using the envelope commands 1733 MAIL FROM: 1734 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1736 or to jkl.org, using the envelope commands 1738 MAIL FROM: 1739 RCPT TO:<@jkl.org:userc@d.bar.org> 1741 Attempting to use relaying this way is now strongly discouraged. 1742 Since hosts are not required to relay mail at all, xyz.com MAY also 1743 reject the message entirely when the RCPT command is received, using 1744 a 550 code (since this is a "policy reason"). 1746 If service extensions were negotiated, the RCPT command may also 1747 carry parameters associated with a particular service extension 1748 offered by the server. The client MUST NOT transmit parameters other 1749 than those associated with a service extension offered by the server 1750 in its EHLO response. 1752 Syntax: 1754 rcpt = "RCPT TO:" ( "" / "" / 1755 Forward-path ) [SP Rcpt-parameters] CRLF 1757 Note that, in a departure from the usual rules for 1758 local-parts, the "Postmaster" string shown above is 1759 treated as case-insensitive. 1761 4.1.1.4. DATA (DATA) 1763 The receiver normally sends a 354 response to DATA, and then treats 1764 the lines (strings ending in sequences, as described in 1765 Section 2.3.7) following the command as mail data from the sender. 1766 This command causes the mail data to be appended to the mail data 1767 buffer. The mail data may contain any of the 128 ASCII character 1768 codes, although experience has indicated that use of control 1769 characters other than SP, HT, CR, and LF may cause problems and 1770 SHOULD be avoided when possible. 1772 The mail data are terminated by a line containing only a period, that 1773 is, the character sequence ".", where the first is 1774 actually the terminator of the previous line (see Section 4.5.2). 1775 This is the end of mail data indication. The first of this 1776 terminating sequence is also the that ends the final line of 1777 the data (message text) or, if there was no mail data, ends the DATA 1778 command itself (the "no mail data" case does not conform to this 1779 specification since it would require that neither the trace header 1780 fields required by this specification nor the message header section 1781 required by RFC 5322 [11] be transmitted). An extra MUST NOT 1782 be added, as that would cause an empty line to be added to the 1783 message. The only exception to this rule would arise if the message 1784 body were passed to the originating SMTP-sender with a final "line" 1785 that did not end in ; in that case, the originating SMTP system 1786 MUST either reject the message as invalid or add in order to 1787 have the receiving SMTP server recognize the "end of data" condition. 1789 The custom of accepting lines ending only in , as a concession to 1790 non-conforming behavior on the part of some UNIX systems, has proven 1791 to cause more interoperability problems than it solves, and SMTP 1792 server systems MUST NOT do this, even in the name of improved 1793 robustness. In particular, the sequence "." (bare line 1794 feeds, without carriage returns) MUST NOT be treated as equivalent to 1795 . as the end of mail data indication. 1797 Receipt of the end of mail data indication requires the server to 1798 process the stored mail transaction information. This processing 1799 consumes the information in the reverse-path buffer, the forward-path 1800 buffer, and the mail data buffer, and on the completion of this 1801 command these buffers are cleared. If the processing is successful, 1802 the receiver MUST send an OK reply. If the processing fails, the 1803 receiver MUST send a failure reply. The SMTP model does not allow 1804 for partial failures at this point: either the message is accepted by 1805 the server for delivery and a positive response is returned or it is 1806 not accepted and a failure reply is returned. In sending a positive 1807 "250 OK" completion reply to the end of data indication, the receiver 1808 takes full responsibility for the message (see Section 6.1). Errors 1809 that are diagnosed subsequently MUST be reported in a mail message, 1810 as discussed in Section 4.4. 1812 When the SMTP server accepts a message either for relaying or for 1813 final delivery, it inserts a trace record (also referred to 1814 interchangeably as a "time stamp line" or "Received" line) at the top 1815 of the mail data. This trace record indicates the identity of the 1816 host that sent the message, the identity of the host that received 1817 the message (and is inserting this time stamp), and the date and time 1818 the message was received. Relayed messages will have multiple time 1819 stamp lines. Details for formation of these lines, including their 1820 syntax, is specified in Section 4.4. 1822 Additional discussion about the operation of the DATA command appears 1823 in Section 3.3. 1825 Syntax: 1827 data = "DATA" CRLF 1829 4.1.1.5. RESET (RSET) 1831 This command specifies that the current mail transaction will be 1832 aborted. Any stored sender, recipients, and mail data MUST be 1833 discarded, and all buffers and state tables cleared. The receiver 1834 MUST send a "250 OK" reply to a RSET command with no arguments. A 1835 reset command may be issued by the client at any time. It is 1836 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1837 immediately after EHLO, before EHLO is issued in the session, after 1838 an end of data indicator has been sent and acknowledged, or 1839 immediately before a QUIT. An SMTP server MUST NOT close the 1840 connection as the result of receiving a RSET; that action is reserved 1841 for QUIT (see Section 4.1.1.10). 1843 Since EHLO implies some additional processing and response by the 1844 server, RSET will normally be more efficient than reissuing that 1845 command, even though the formal semantics are the same. 1847 There are circumstances, contrary to the intent of this 1848 specification, in which an SMTP server may receive an indication that 1849 the underlying TCP connection has been closed or reset. To preserve 1850 the robustness of the mail system, SMTP servers SHOULD be prepared 1851 for this condition and SHOULD treat it as if a QUIT had been received 1852 before the connection disappeared. 1854 Syntax: 1856 rset = "RSET" CRLF 1858 4.1.1.6. VERIFY (VRFY) 1860 This command asks the receiver to confirm that the argument 1861 identifies a user or mailbox. If it is a user name, information is 1862 returned as specified in Section 3.5. 1864 This command has no effect on the reverse-path buffer, the forward- 1865 path buffer, or the mail data buffer. 1867 Syntax: 1869 vrfy = "VRFY" SP String CRLF 1871 4.1.1.7. EXPAND (EXPN) 1873 This command asks the receiver to confirm that the argument 1874 identifies a mailing list, and if so, to return the membership of 1875 that list. If the command is successful, a reply is returned 1876 containing information as described in Section 3.5. This reply will 1877 have multiple lines except in the trivial case of a one-member list. 1879 This command has no effect on the reverse-path buffer, the forward- 1880 path buffer, or the mail data buffer, and it may be issued at any 1881 time. 1883 Syntax: 1885 expn = "EXPN" SP String CRLF 1887 4.1.1.8. HELP (HELP) 1889 This command causes the server to send helpful information to the 1890 client. The command MAY take an argument (e.g., any command name) 1891 and return more specific information as a response. 1893 This command has no effect on the reverse-path buffer, the forward- 1894 path buffer, or the mail data buffer, and it may be issued at any 1895 time. 1897 SMTP servers SHOULD support HELP without arguments and MAY support it 1898 with arguments. 1900 Syntax: 1902 help = "HELP" [ SP String ] CRLF 1904 4.1.1.9. NOOP (NOOP) 1906 This command does not affect any parameters or previously entered 1907 commands. It specifies no action other than that the receiver send a 1908 "250 OK" reply. 1910 This command has no effect on the reverse-path buffer, the forward- 1911 path buffer, or the mail data buffer, and it may be issued at any 1912 time. If a parameter string is specified, servers SHOULD ignore it. 1914 Syntax: 1916 noop = "NOOP" [ SP String ] CRLF 1918 4.1.1.10. QUIT (QUIT) 1920 This command specifies that the receiver MUST send a "221 OK" reply, 1921 and then close the transmission channel. 1923 The receiver MUST NOT intentionally close the transmission channel 1924 until it receives and replies to a QUIT command (even if there was an 1925 error). The sender MUST NOT intentionally close the transmission 1926 channel until it sends a QUIT command, and it SHOULD wait until it 1927 receives the reply (even if there was an error response to a previous 1928 command). If the connection is closed prematurely due to violations 1929 of the above or system or network failure, the server MUST cancel any 1930 pending transaction, but not undo any previously completed 1931 transaction, and generally MUST act as if the command or transaction 1932 in progress had received a temporary error (i.e., a 4yz response). 1934 The QUIT command may be issued at any time. Any current uncompleted 1935 mail transaction will be aborted. 1937 Syntax: 1939 quit = "QUIT" CRLF 1941 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1943 If the server SMTP does not recognize or cannot implement one or more 1944 of the parameters associated with a particular MAIL FROM or RCPT TO 1945 command, it will return code 555. 1947 If, for some reason, the server is temporarily unable to accommodate 1948 one or more of the parameters associated with a MAIL FROM or RCPT TO 1949 command, and if the definition of the specific parameter does not 1950 mandate the use of another code, it should return code 455. 1952 Errors specific to particular parameters and their values will be 1953 specified in the parameter's defining RFC. 1955 4.1.2. Command Argument Syntax 1957 The syntax of the argument clauses of the above commands (using the 1958 syntax specified in RFC 5234 [10] where applicable) is given below. 1959 Some of the productions given below are used only in conjunction with 1960 source routes as described in Appendix C. Some terminals not defined 1961 in this document, but are defined elsewhere, specifically: 1963 In the "core" syntax in Appendix B of RFC 5234 [10]: ALPHA , CRLF 1964 , DIGIT , HEXDIG , and SP 1966 In the message format syntax in RFC 5322 [11]: atext , CFWS , and 1967 FWS . 1969 Reverse-path = Path / "<>" 1971 Forward-path = Path 1973 Path = "<" [ A-d-l ":" ] Mailbox ">" 1975 A-d-l = At-domain *( "," At-domain ) 1976 ; Note that this form, the so-called "source 1977 ; route", MUST BE accepted, SHOULD NOT be 1978 ; generated, and SHOULD be ignored. 1980 At-domain = "@" Domain 1982 Mail-parameters = esmtp-param *(SP esmtp-param) 1984 Rcpt-parameters = esmtp-param *(SP esmtp-param) 1986 esmtp-param = esmtp-keyword ["=" esmtp-value] 1988 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1990 esmtp-value = 1*(%d33-60 / %d62-126) 1991 ; any CHAR excluding "=", SP, and control 1992 ; characters. If this string is an email address, 1993 ; i.e., a Mailbox, then the "xtext" syntax [33] 1994 ; SHOULD be used. 1996 Keyword = Ldh-str 1998 Argument = Atom 1999 Domain = sub-domain *("." sub-domain) 2001 sub-domain = Let-dig [Ldh-str] 2003 Let-dig = ALPHA / DIGIT 2005 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2007 address-literal = "[" ( IPv4-address-literal / 2008 IPv6-address-literal / 2009 General-address-literal ) "]" 2010 ; See Section 4.1.3 2012 Mailbox = Local-part "@" ( Domain / address-literal ) 2014 Local-part = Dot-string / Quoted-string 2015 ; MAY be case-sensitive 2017 Dot-string = Atom *("." Atom) 2019 Atom = 1*atext 2021 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2023 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2025 quoted-pairSMTP = %d92 %d32-126 2026 ; i.e., backslash followed by any ASCII 2027 ; graphic (including itself) or SPace 2029 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2030 ; i.e., within a quoted string, any 2031 ; ASCII graphic or space is permitted 2032 ; without blackslash-quoting except 2033 ; double-quote and the backslash itself. 2035 String = Atom / Quoted-string 2037 While the above definition for Local-part is relatively permissive, 2038 for maximum interoperability, a host that expects to receive mail 2039 SHOULD avoid defining mailboxes where the Local-part requires (or 2040 uses) the Quoted-string form or where the Local-part is case- 2041 sensitive. For any purposes that require generating or comparing 2042 Local-parts (e.g., to specific mailbox names), all quoted forms MUST 2043 be treated as equivalent, and the sending system SHOULD transmit the 2044 form that uses the minimum quoting possible. 2046 Systems MUST NOT define mailboxes in such a way as to require the use 2047 in SMTP of non-ASCII characters (octets with the high order bit set 2048 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2049 These characters MUST NOT be used in MAIL or RCPT commands or other 2050 commands that require mailbox names. 2052 Note that the backslash, "\", is a quote character, which is used to 2053 indicate that the next character is to be used literally (instead of 2054 its normal interpretation). For example, "Joe\,Smith" indicates a 2055 single nine-character user name string with the comma being the 2056 fourth character of that string. 2058 To promote interoperability and consistent with long-standing 2059 guidance about conservative use of the DNS in naming and applications 2060 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2061 characters outside the set of alphabetic characters, digits, and 2062 hyphen MUST NOT appear in domain name labels for SMTP clients or 2063 servers. In particular, the underscore character is not permitted. 2064 SMTP servers that receive a command in which invalid character codes 2065 have been employed, and for which there are no other reasons for 2066 rejection, MUST reject that command with a 501 response (this rule, 2067 like others, could be overridden by appropriate SMTP extensions). 2069 4.1.3. Address Literals 2071 Sometimes a host is not known to the domain name system and 2072 communication (and, in particular, communication to report and repair 2073 the error) is blocked. To bypass this barrier, a special literal 2074 form of the address is allowed as an alternative to a domain name. 2075 For IPv4 addresses, this form uses four small decimal integers 2076 separated by dots and enclosed by brackets such as [123.255.37.2], 2077 which indicates an (IPv4) Internet Address in sequence-of-octets 2078 form. For IPv6 and other forms of addressing that might eventually 2079 be standardized, the form consists of a standardized "tag" that 2080 identifies the address syntax, a colon, and the address itself, in a 2081 format specified as part of the relevant standards (i.e., RFC 4291 2082 [9] for IPv6). 2083 [[CREF11: [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet 2084 another modification to the IPv6 address literal syntax, based on 2085 part on RFC 5952. We should consider whether those, or other, 2086 modifications are appropriate and/or whether, given both the issues 2087 of spam/malware and servers supporting multiple domains, it it time 2088 to deprecate mailboxes containing address literals entirely (EHLO 2089 fields may be a different issue). If we are going to allow IPv6 2090 address literals, it may be time to incorporate something by 2091 reference rather than including specific syntax here (RFC 5952 is 14 2092 pages long and does not contain any ABNF).]] 2093 Specifically: 2095 IPv4-address-literal = Snum 3("." Snum) 2097 IPv6-address-literal = "IPv6:" IPv6-addr 2099 General-address-literal = Standardized-tag ":" 1*dcontent 2101 Standardized-tag = Ldh-str 2102 ; Standardized-tag MUST be specified in a 2103 ; Standards-Track RFC and registered with IANA 2105 dcontent = %d33-90 / ; Printable US-ASCII 2106 %d94-126 ; excl. "[", "\", "]" 2108 Snum = 1*3DIGIT 2109 ; representing a decimal integer 2110 ; value in the range 0 through 255 2112 IPv6-addr = 6( h16 ":" ) ls32 2113 / "::" 5( h16 ":" ) ls32 2114 / [ h16 ] "::" 4( h16 ":" ) ls32 2115 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2116 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2117 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2118 / [ *4( h16 ":" ) h16 ] "::" ls32 2119 / [ *5( h16 ":" ) h16 ] "::" h16 2120 / [ *6( h16 ":" ) h16 ] "::" 2121 ; This definition is consistent with the one for 2122 ; URIs [40]. 2124 ls32 = ( h16 ":" h16 ) / IPv4address 2125 ; least-significant 32 bits of address 2127 h16 = 1*4HEXDIG 2128 ; 16 bits of address represented in hexadecimal 2129 [[CREF12: [5321bis](2821ter) 2821bis Last Call 2130 comment]] 2132 4.1.4. Order of Commands 2134 There are restrictions on the order in which these commands may be 2135 used. 2137 A session that will contain mail transactions MUST first be 2138 initialized by the use of the EHLO command. An SMTP server SHOULD 2139 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2140 without this initialization. 2142 An EHLO command MAY be issued by a client later in the session. If 2143 it is issued after the session begins and the EHLO command is 2144 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2145 and reset the state exactly as if a RSET command had been issued. In 2146 other words, the sequence of RSET followed immediately by EHLO is 2147 redundant, but not harmful other than in the performance cost of 2148 executing unnecessary commands. 2150 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2151 502, or 550 failure replies MUST be returned as appropriate. The 2152 SMTP server MUST stay in the same state after transmitting these 2153 replies that it was in before the EHLO was received. 2155 The SMTP client MUST, if possible, ensure that the domain parameter 2156 to the EHLO command is a primary host name as specified for this 2157 command in Section 2.3.5. If this is not possible (e.g., when the 2158 client's address is dynamically assigned and the client does not have 2159 an obvious name), an address literal SHOULD be substituted for the 2160 domain name. 2162 An SMTP server MAY verify that the domain name argument in the EHLO 2163 command actually corresponds to the IP address of the client. 2164 [[CREF13: [5321bis] [[Note in draft -- proposed change to "An SMTP 2165 server MAY verify that the domain name argument in the EHLO command 2166 has an address record matching the IP address of the client." --David 2167 MacQuigg, david_macquigg@yahoo.com, Friday, 20090130 0637 -0700]]]] 2168 However, if the verification fails, the server MUST NOT refuse to 2169 accept a message on that basis. Information captured in the 2170 verification attempt is for logging and tracing purposes. Note that 2171 this prohibition applies to the matching of the parameter to its IP 2172 address only; see Section 7.9 for a more extensive discussion of 2173 rejecting incoming connections or mail messages. 2175 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2176 during a session, or without previously initializing a session. SMTP 2177 servers SHOULD process these normally (that is, not return a 503 2178 code) even if no EHLO command has yet been received; clients SHOULD 2179 open a session with EHLO before sending these commands. 2181 If these rules are followed, the example in RFC 821 that shows "550 2182 access denied to you" in response to an EXPN command is incorrect 2183 unless an EHLO command precedes the EXPN or the denial of access is 2184 based on the client's IP address or other authentication or 2185 authorization-determining mechanisms. 2187 The MAIL command (or the obsolete SEND, SOML, or SAML commands) 2189 [[5321bis Editor's Note: is there any reason to not clean those 2190 commands out of this entirely, replacing them with, e.g., a strong 2191 reference to Appendix F.6]] 2192 begins a mail transaction. Once started, a mail transaction consists 2193 of a transaction beginning command, one or more RCPT commands, and a 2194 DATA command, in that order. A mail transaction may be aborted by 2195 the RSET, a new EHLO, or the QUIT command. There may be zero or more 2196 transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be 2197 sent if a mail transaction is already open, i.e., it should be sent 2198 only if no mail transaction had been started in the session, or if 2199 the previous one successfully concluded with a successful DATA 2200 command, or if the previous one was aborted, e.g., with a RSET or new 2201 EHLO. [[CREF14: [5321bis] 2821ter note: see comment about changing 2202 this convoluted discussion to talk about 'mail transaction' above. 2203 --Jck]] 2205 If the transaction beginning command argument is not acceptable, a 2206 501 failure reply MUST be returned and the SMTP server MUST stay in 2207 the same state. If the commands in a transaction are out of order to 2208 the degree that they cannot be processed by the server, a 503 failure 2209 reply MUST be returned and the SMTP server MUST stay in the same 2210 state. 2212 The last command in a session MUST be the QUIT command. The QUIT 2213 command SHOULD be used by the client SMTP to request connection 2214 closure, even when no session opening command was sent and accepted. 2216 4.1.5. Private-Use Commands 2218 As specified in Section 2.2.2, commands starting in "X" may be used 2219 by bilateral agreement between the client (sending) and server 2220 (receiving) SMTP agents. An SMTP server that does not recognize such 2221 a command is expected to reply with "500 Command not recognized". An 2222 extended SMTP server MAY list the feature names associated with these 2223 private commands in the response to the EHLO command. 2225 Commands sent or accepted by SMTP systems that do not start with "X" 2226 MUST conform to the requirements of Section 2.2.2. 2228 4.2. SMTP Replies 2230 Replies to SMTP commands serve to ensure the synchronization of 2231 requests and actions in the process of mail transfer and to guarantee 2232 that the SMTP client always knows the state of the SMTP server. 2233 Every command MUST generate exactly one reply. 2235 The details of the command-reply sequence are described in 2236 Section 4.3. 2238 An SMTP reply consists of a three digit number (transmitted as three 2239 numeric characters) followed by some text unless specified otherwise 2240 in this document. The number is for use by automata to determine 2241 what state to enter next; the text is for the human user. The three 2242 digits contain enough encoded information that the SMTP client need 2243 not examine the text and may either discard it or pass it on to the 2244 user, as appropriate. Exceptions are as noted elsewhere in this 2245 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2246 are associated with message text that must be parsed and interpreted 2247 by machines. In the general case, the text may be receiver dependent 2248 and context dependent, so there are likely to be varying texts for 2249 each reply code. A discussion of the theory of reply codes is given 2250 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2251 three-digit code, , one line of text, and , or a multiline 2252 reply (as defined in the same section). Since, in violation of this 2253 specification, the text is sometimes not sent, clients that do not 2254 receive it SHOULD be prepared to process the code alone (with or 2255 without a trailing space character). Only the EHLO, EXPN, and HELP 2256 commands are expected to result in multiline replies in normal 2257 circumstances; however, multiline replies are allowed for any 2258 command. 2260 In ABNF, server responses are: 2262 Greeting = ( "220 " (Domain / address-literal) 2263 [ SP textstring ] CRLF ) / 2264 ( "220-" (Domain / address-literal) 2265 [ SP textstring ] CRLF 2266 *( "220-" [ textstring ] CRLF ) 2267 "220" [ SP textstring ] CRLF ) 2269 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2271 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2272 Reply-code [ SP textstring ] CRLF 2274 Reply-code = %x32-35 %x30-35 %x30-39 2276 where "Greeting" appears only in the 220 response that announces that 2277 the server is opening its part of the connection. (Other possible 2278 server responses upon connection follow the syntax of Reply-line.) 2280 An SMTP server SHOULD send only the reply codes listed in this 2281 document or additions to the list as discussed below. 2282 [[CREF15: [5321bis] 20140804: New text to clear up ambiguity.]] 2283 An SMTP server SHOULD use the text shown in the examples whenever 2284 appropriate. 2286 An SMTP client MUST determine its actions only by the reply code, not 2287 by the text (except for the "change of address" 251 and 551 and, if 2288 necessary, 220, 221, and 421 replies); in the general case, any text, 2289 including no text at all (although senders SHOULD NOT send bare 2290 codes), MUST be acceptable. The space (blank) following the reply 2291 code is considered part of the text. Whenever possible, a sender- 2292 SMTP SHOULD test the first digit (severity indication) of a reply 2293 code it receives. 2294 [[CREF16: [5321bis] 20141209 [[Note in Draft: What is that sentence 2295 supposed to be tell us? Test the first digit and examine the others 2296 only if necessary? Note the interaction between this and various 2297 flaps about adding new codes.]]]] 2299 The list of codes that appears below MUST NOT be construed as 2300 permanent. While the addition of new codes should be a rare and 2301 significant activity, with supplemental information in the textual 2302 part of the response (including enhanced status codes [34] and the 2303 successors to that specification) 2304 [[CREF17: [5321bis] 20140802: New text for clarity]] 2305 being preferred, new codes may be added as the result of new 2306 Standards or Standards-Track specifications. Consequently, a sender- 2307 SMTP MUST be prepared to handle codes not specified in this document 2308 and MUST do so by interpreting the first digit only. 2310 In the absence of extensions negotiated with the client, SMTP servers 2311 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2312 or 5. Clients that receive such out-of-range codes SHOULD normally 2313 treat them as fatal errors and terminate the mail transaction. 2315 4.2.1. Reply Code Severities and Theory 2317 The three digits of the reply each have a special significance. The 2318 first digit denotes whether the response is good, bad, or incomplete. 2319 An unsophisticated SMTP client, or one that receives an unexpected 2320 code, will be able to determine its next action (proceed as planned, 2321 redo, retrench, etc.) by examining this first digit. An SMTP client 2322 that wants to know approximately what kind of error occurred (e.g., 2323 mail system error, command syntax error) may examine the second 2324 digit. The third digit and any supplemental information that may be 2325 present is reserved for the finest gradation of information. 2327 There are four values for the first digit of the reply code: 2329 2yz Positive Completion reply 2330 The requested action has been successfully completed. A new 2331 request may be initiated. 2333 3yz Positive Intermediate reply 2334 The command has been accepted, but the requested action is being 2335 held in abeyance, pending receipt of further information. The 2336 SMTP client should send another command specifying this 2337 information. This reply is used in command sequence groups (i.e., 2338 in DATA). 2340 4yz Transient Negative Completion reply 2341 The command was not accepted, and the requested action did not 2342 occur. However, the error condition is temporary, and the action 2343 may be requested again. The sender should return to the beginning 2344 of the command sequence (if any). It is difficult to assign a 2345 meaning to "transient" when two different sites (receiver- and 2346 sender-SMTP agents) must agree on the interpretation. Each reply 2347 in this category might have a different time value, but the SMTP 2348 client SHOULD try again. A rule of thumb to determine whether a 2349 reply fits into the 4yz or the 5yz category (see below) is that 2350 replies are 4yz if they can be successful if repeated without any 2351 change in command form or in properties of the sender or receiver 2352 (that is, the command is repeated identically and the receiver 2353 does not put up a new implementation). 2355 5yz Permanent Negative Completion reply 2356 The command was not accepted and the requested action did not 2357 occur. The SMTP client SHOULD NOT repeat the exact request (in 2358 the same sequence). Even some "permanent" error conditions can be 2359 corrected, so the human user may want to direct the SMTP client to 2360 reinitiate the command sequence by direct action at some point in 2361 the future (e.g., after the spelling has been changed, or the user 2362 has altered the account status). 2364 It is worth noting that the file transfer protocol (FTP) [14] uses a 2365 very similar code architecture and that the SMTP codes are based on 2366 the FTP model. However, SMTP uses a one-command, one-response model 2367 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2368 SMTP model. 2370 The second digit encodes responses in specific categories: 2372 x0z Syntax: These replies refer to syntax errors, syntactically 2373 correct commands that do not fit any functional category, and 2374 unimplemented or superfluous commands. 2376 x1z Information: These are replies to requests for information, such 2377 as status or help. 2379 x2z Connections: These are replies referring to the transmission 2380 channel. 2382 x3z Unspecified. 2384 x4z Unspecified. 2386 x5z Mail system: These replies indicate the status of the receiver 2387 mail system vis-a-vis the requested transfer or other mail system 2388 action. 2390 The third digit gives a finer gradation of meaning in each category 2391 specified by the second digit. The list of replies illustrates this. 2392 Each reply text is recommended rather than mandatory, and may even 2393 change according to the command with which it is associated. On the 2394 other hand, the reply codes must strictly follow the specifications 2395 in this section. Receiver implementations should not invent new 2396 codes for slightly different situations from the ones described here, 2397 but rather adapt codes already defined. 2399 For example, a command such as NOOP, whose successful execution does 2400 not offer the SMTP client any new information, will return a 250 2401 reply. The reply is 502 when the command requests an unimplemented 2402 non-site-specific action. A refinement of that is the 504 reply for 2403 a command that is implemented, but that requests an unimplemented 2404 parameter. 2406 The reply text may be longer than a single line; in these cases the 2407 complete text must be marked so the SMTP client knows when it can 2408 stop reading the reply. This requires a special format to indicate a 2409 multiple line reply. 2411 The format for multiline replies requires that every line, except the 2412 last, begin with the reply code, followed immediately by a hyphen, 2413 "-" (also known as minus), followed by text. The last line will 2414 begin with the reply code, followed immediately by , optionally 2415 some text, and . As noted above, servers SHOULD send the 2416 if subsequent text is not sent, but clients MUST be prepared for it 2417 to be omitted. 2419 For example: 2421 250-First line 2422 250-Second line 2423 250-234 Text beginning with numbers 2424 250 The last line 2426 In a multiline reply, the reply code on each of the lines MUST be the 2427 same. It is reasonable for the client to rely on this, so it can 2428 make processing decisions based on the code in any line, assuming 2429 that all others will be the same. In a few cases, there is important 2430 data for the client in the reply "text". The client will be able to 2431 identify these cases from the current context. 2433 4.2.2. Reply Codes by Function Groups 2435 500 Syntax error, command unrecognized (This may include errors such 2436 as command line too long) 2438 501 Syntax error in parameters or arguments 2440 502 Command not implemented (see Section 4.2.4.1) 2442 503 Bad sequence of commands 2444 504 Command parameter not implemented 2446 211 System status, or system help reply 2448 214 Help message (Information on how to use the receiver or the 2449 meaning of a particular non-standard command; this reply is useful 2450 only to the human user) 2452 220 Service ready 2454 221 Service closing transmission channel 2456 421 Service not available, closing transmission channel 2457 (This may be a reply to any command if the service knows it must 2458 shut down) 2460 hangText="521"> No mail service here. [[CREF18: 2461 [5321bis]20140804: Specific code introduced with RFC 1846, updated 2462 and specified in draft-klensin-smtp-521code.]] 2464 556 No mail service at this domain. [[CREF19: [5321bis] 20140912: 2465 Specific code introduced in draft-klensin-smtp-521code-02 (RFC 2466 7504), largely for nullMX]] 2468 250 Requested mail action okay, completed 2470 251 User not local; will forward to (See Section 3.4) 2472 252 Cannot VRFY user, but will accept message and attempt delivery 2473 (See Section 3.5.3) 2475 455 Server unable to accommodate parameters 2477 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2479 450 Requested mail action not taken: mailbox unavailable (e.g., 2480 mailbox busy or temporarily blocked for policy reasons) 2482 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2483 not found, no access, or command rejected for policy reasons) 2485 451 Requested action aborted: error in processing 2487 551 User not local; please try (See Section 3.4) 2489 452 Requested action not taken: insufficient system storage 2491 552 Requested mail action aborted: exceeded storage allocation 2493 553 Requested action not taken: mailbox name not allowed (e.g., 2494 mailbox syntax incorrect) 2496 354 Start mail input; end with . 2498 554 Transaction failed (Or, in the case of a connection-opening 2499 response, "No SMTP service here") 2500 [[CREF20: [5321bis] [[Note in Draft: Revise above statement in the 2501 light of new 521 code??]] ]] 2503 4.2.3. Reply Codes in Numeric Order 2505 211 System status, or system help reply 2507 214 Help message (Information on how to use the receiver or the 2508 meaning of a particular non-standard command; this reply is useful 2509 only to the human user) 2511 220 Service ready 2513 221 Service closing transmission channel 2515 250 Requested mail action okay, completed 2517 251 User not local; will forward to (See Section 3.4) 2519 252 Cannot VRFY user, but will accept message and attempt delivery 2520 (See Section 3.5.3) 2522 354 Start mail input; end with . 2524 421 Service not available, closing transmission channel 2525 (This may be a reply to any command if the service knows it must 2526 shut down) 2528 450 Requested mail action not taken: mailbox unavailable (e.g., 2529 mailbox busy or temporarily blocked for policy reasons) 2531 451 Requested action aborted: local error in processing 2533 452 Requested action not taken: insufficient system storage 2535 455 Server unable to accommodate parameters 2537 500 Syntax error, command unrecognized (This may include errors such 2538 as command line too long) 2540 501 Syntax error in parameters or arguments 2542 502 Command not implemented (see Section 4.2.4.1) 2544 503 Bad sequence of commands 2546 504 Command parameter not implemented 2548 521 No mail service 2550 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2551 not found, no access, or command rejected for policy reasons) 2553 551 User not local; please try (See Section 3.4) 2555 552 Requested mail action aborted: exceeded storage allocation 2557 553 Requested action not taken: mailbox name not allowed (e.g., 2558 mailbox syntax incorrect) 2560 554 Transaction failed (Or, in the case of a connection-opening 2561 response, "No SMTP service here") 2563 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2565 556 No mail service at this domain. 2567 4.2.4. Some specific code situations and relationships 2569 4.2.4.1. Reply Code 502 2571 Questions have been raised as to when reply code 502 (Command not 2572 implemented) SHOULD be returned in preference to other codes. 502 2573 SHOULD be used when the command is actually recognized by the SMTP 2574 server, but not implemented. If the command is not recognized, code 2575 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2576 capabilities in response to EHLO for which they will return 502 (or 2577 500) replies. 2579 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2581 [[CREF21: [5321bis] This section is new with 5321bis. ]] 2583 Codes 521, 554, and 556 are all used to report different types of "no 2584 mail accepted" situations. They differ as follows. 521 is an 2585 indication from a system answering on the SMTP port that it does not 2586 support SMTP service (a so-called "dummy server" as discussed in RFC 2587 1846 [19] and elsewhere). Obviously, it requires that system exist 2588 and that a connection can be made successfully to it. Because a 2589 system that does not accept any mail cannot meaningfully accept a 2590 RCPT command, any commands (other than QUIT) issued after an SMTP 2591 server has issued a 521 reply are client (sender) errors. 556 is 2592 used by a message submission or intermediate SMTP system (see 2593 Section 1.1) to report that it cannot forward the message further 2594 because it knows (e.g., from a DNS entry [51]) that the recipient 2595 domain does not accept mail. It would normally be used in response 2596 to a RCPT or similar (extension) command when the SMTP system 2597 identifies a domain that it can (or has) determined never accepts 2598 mail. Other codes, including 554 and the temporary 450, are used for 2599 more transient situations and situations in which an SMTP server 2600 cannot or will not deliver to (or accept mail for) a particular 2601 system or mailbox for policy reasons rather than ones directly 2602 related to SMTP processing. 2604 4.2.4.3. Reply Codes after DATA and the Subsequent . 2606 When an SMTP server returns a positive completion status (2yz code) 2607 after the DATA command is completed with ., it accepts 2608 responsibility for: 2610 o delivering the message (if the recipient mailbox exists), or 2612 o if attempts to deliver the message fail due to transient 2613 conditions, retrying delivery some reasonable number of times at 2614 intervals as specified in Section 4.5.4. 2616 o if attempts to deliver the message fail due to permanent 2617 conditions, or if repeated attempts to deliver the message fail 2618 due to transient conditions, returning appropriate notification to 2619 the sender of the original message (using the address in the SMTP 2620 MAIL command). 2622 When an SMTP server returns a temporary error status (4yz) code after 2623 the DATA command is completed with ., it MUST NOT make a 2624 subsequent attempt to deliver that message. The SMTP client retains 2625 responsibility for the delivery of that message and may either return 2626 it to the user or requeue it for a subsequent attempt (see 2627 Section 4.5.4.1). 2629 The user who originated the message SHOULD be able to interpret the 2630 return of a transient failure status (by mail message or otherwise) 2631 as a non-delivery indication, just as a permanent failure would be 2632 interpreted. If the client SMTP successfully handles these 2633 conditions, the user will not receive such a reply. 2635 When an SMTP server returns a permanent error status (5yz) code after 2636 the DATA command is completed with ., it MUST NOT make 2637 any subsequent attempt to deliver the message. As with temporary 2638 error status codes, the SMTP client retains responsibility for the 2639 message, but SHOULD NOT again attempt delivery to the same server 2640 without user review of the message and response and appropriate 2641 intervention. 2643 4.3. Sequencing of Commands and Replies 2645 4.3.1. Sequencing Overview 2647 The communication between the sender and receiver is an alternating 2648 dialogue, controlled by the sender. As such, the sender issues a 2649 command and the receiver responds with a reply. Unless other 2650 arrangements are negotiated through service extensions, the sender 2651 MUST wait for this response before sending further commands. One 2652 important reply is the connection greeting. Normally, a receiver 2653 will send a 220 "Service ready" reply when the connection is 2654 completed. The sender SHOULD wait for this greeting message before 2655 sending any commands. 2657 Note: all the greeting-type replies have the official name (the 2658 fully-qualified primary domain name) of the server host as the first 2659 word following the reply code. Sometimes the host will have no 2660 meaningful name. See Section 4.1.3 for a discussion of alternatives 2661 in these situations. 2663 For example, 2664 220 ISIF.USC.EDU Service ready 2666 or 2668 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2670 or 2672 220 [10.0.0.1] Clueless host service ready 2674 The table below lists alternative success and failure replies for 2675 each command. These SHOULD be strictly adhered to. A receiver MAY 2676 substitute text in the replies, but the meanings and actions implied 2677 by the code numbers and by the specific command reply sequence MUST 2678 be preserved. However, in order to provide robustness as SMTP is 2679 extended and evolves, the discussion in Section 4.2.1 still applies: 2680 all SMTP clients MUST be prepared to accept any code that conforms to 2681 the discussion in that section and MUST be prepared to interpret it 2682 on the basis of its first digit only. [[CREF22: [5321bis] 20140914: 2683 Above sentence is new text based on yet another round of discussions 2684 about "invalid codes".]] 2686 4.3.2. Command-Reply Sequences 2688 Each command is listed with its usual possible replies. The prefixes 2689 used before the possible replies are "I" for intermediate, "S" for 2690 success, and "E" for error. Since some servers may generate other 2691 replies under special circumstances, and to allow for future 2692 extension, SMTP clients SHOULD, when possible, interpret only the 2693 first digit of the reply and MUST be prepared to deal with 2694 unrecognized reply codes by interpreting the first digit only. 2695 Unless extended using the mechanisms described in Section 2.2, SMTP 2696 servers MUST NOT transmit reply codes to an SMTP client that are 2697 other than three digits or that do not start in a digit between 2 and 2698 5 inclusive. 2700 These sequencing rules and, in principle, the codes themselves, can 2701 be extended or modified by SMTP extensions offered by the server and 2702 accepted (requested) by the client. However, if the target is more 2703 precise granularity in the codes, rather than codes for completely 2704 new purposes, the system described in RFC 3463 [34] SHOULD be used in 2705 preference to the invention of new codes. 2707 In addition to the codes listed below, any SMTP command can return 2708 any of the following codes if the corresponding unusual circumstances 2709 are encountered: 2711 500 For the "command line too long" case or if the command name was 2712 not recognized. Note that producing a "command not recognized" 2713 error in response to the required subset of these commands is a 2714 violation of this specification. Similarly, producing a "command 2715 too long" message for a command line shorter than 512 characters 2716 would violate the provisions of Section 4.5.3.1.4. 2718 501 Syntax error in command or arguments. In order to provide for 2719 future extensions, commands that are specified in this document as 2720 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2721 message if arguments are supplied in the absence of EHLO- 2722 advertised extensions. 2724 421 Service shutting down and closing transmission channel 2726 Specific sequences are: 2728 CONNECTION ESTABLISHMENT 2730 S: 220 2731 E: 521, 554 2733 EHLO or HELO 2735 S: 250 2736 E: 504 (a conforming implementation could return this code only 2737 in fairly obscure cases), 550, 502 (permitted only with an old- 2738 style server that does not support EHLO) 2740 MAIL 2742 S: 250 2743 E: 552, 451, 452, 550, 553, 503, 455, 555 2745 RCPT 2747 S: 250, 251 (but see Section 3.4 for discussion of 251 and 551) 2748 E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555 2750 DATA 2752 I: 354 -> data -> S: 250 2754 E: 552, 554, 451, 452 2756 E: 450, 550 (rejections for policy reasons) 2758 E: 503, 554 2760 RSET 2762 S: 250 2764 VRFY 2766 S: 250, 251, 252 2767 E: 550, 551, 553, 502, 504 2769 EXPN 2771 S: 250, 252 2772 E: 550, 500, 502, 504 2774 HELP 2776 S: 211, 214 2777 E: 502, 504 2779 NOOP 2781 S: 250 2783 QUIT 2785 S: 221 2787 4.4. Trace Information 2789 When an SMTP server receives a message for delivery or further 2790 processing, it MUST insert trace (often referred to as "time stamp" 2791 or "Received" information) [[CREF23: [5321bis] See note on 2792 rfc5321bis-00c above]] information at the beginning of the message 2793 content, as discussed in Section 4.1.1.4. 2795 This line MUST be structured as follows: 2797 o The FROM clause, which MUST be supplied in an SMTP environment, 2798 SHOULD contain both (1) the name of the source host as presented 2799 in the EHLO command and (2) an address literal containing the IP 2800 address of the source, determined from the TCP connection. 2802 o The ID clause MAY contain an "@" as suggested in RFC 822, but this 2803 is not required. 2805 o If the FOR clause appears, it MUST contain exactly one 2806 entry, even when multiple RCPT commands have been given. Multiple 2807 s raise some security issues and have been deprecated, see 2808 Section 7.2. 2810 An Internet mail program MUST NOT change or delete a Received: line 2811 that was previously added to the message header section. SMTP 2812 servers MUST prepend Received lines to messages; they MUST NOT change 2813 the order of existing lines or insert Received lines in any other 2814 location. 2816 As the Internet grows, comparability of Received header fields is 2817 important for detecting problems, especially slow relays. SMTP 2818 servers that create Received header fields SHOULD use explicit 2819 offsets in the dates (e.g., -0800), rather than time zone names of 2820 any type. Local time (with an offset) SHOULD be used rather than UT 2821 when feasible. This formulation allows slightly more information 2822 about local circumstances to be specified. If UT is needed, the 2823 receiver need merely do some simple arithmetic to convert the values. 2824 Use of UT loses information about the time zone-location of the 2825 server. If it is desired to supply a time zone name, it SHOULD be 2826 included in a comment. 2828 When the delivery SMTP server makes the "final delivery" of a 2829 message, it inserts a return-path line at the beginning of the mail 2830 data. This use of return-path is required; mail systems MUST support 2831 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2833 has left the SMTP environment. Normally, this would mean it had been 2834 delivered to the destination user or an associated mail drop, but in 2835 some cases it may be further processed and transmitted by another 2836 mail system. 2838 It is possible for the mailbox in the return path to be different 2839 from the actual sender's mailbox, for example, if error responses are 2840 to be delivered to a special error handling mailbox rather than to 2841 the message sender. When mailing lists are involved, this 2842 arrangement is common and useful as a means of directing errors to 2843 the list maintainer rather than the message originator. 2845 The text above implies that the final mail data will begin with a 2846 return path line, followed by one or more time stamp lines. These 2847 lines will be followed by the rest of the mail data: first the 2848 balance of the mail header section and then the body (RFC 5322 [11]). 2850 It is sometimes difficult for an SMTP server to determine whether or 2851 not it is making final delivery since forwarding or other operations 2852 may occur after the message is accepted for delivery. Consequently, 2853 any further (forwarding, gateway, or relay) systems MAY remove the 2854 return path and rebuild the MAIL command as needed to ensure that 2855 exactly one such line appears in a delivered message. 2857 A message-originating SMTP system SHOULD NOT send a message that 2858 already contains a Return-path header field. SMTP servers performing 2859 a relay function MUST NOT inspect the message data, and especially 2860 not to the extent needed to determine if Return-path header fields 2861 are present. SMTP servers making final delivery MAY remove Return- 2862 path header fields before adding their own. 2864 The primary purpose of the Return-path is to designate the address to 2865 which messages indicating non-delivery or other mail system failures 2866 are to be sent. For this to be unambiguous, exactly one return path 2867 SHOULD be present when the message is delivered. Systems using RFC 2868 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2869 address, associated with the transport envelope, to which error 2870 reports (e.g., non-delivery messages) should be sent. 2872 Historical note: Text in RFC 822 that appears to contradict the use 2873 of the Return-path header field (or the envelope reverse-path address 2874 from the MAIL command) as the destination for error messages is not 2875 applicable on the Internet. The reverse-path address (as copied into 2876 the Return-path) MUST be used as the target of any mail containing 2877 delivery error messages. 2879 In particular: 2881 o a gateway from SMTP -> elsewhere SHOULD insert a return-path 2882 header field, unless it is known that the "elsewhere" transport 2883 also uses Internet domain addresses and maintains the envelope 2884 sender address separately. 2886 o a gateway from elsewhere -> SMTP SHOULD delete any return-path 2887 header field present in the message, and either copy that 2888 information to the SMTP envelope or combine it with information 2889 present in the envelope of the other transport system to construct 2890 the reverse-path argument to the MAIL command in the SMTP 2891 envelope. 2893 The server must give special treatment to cases in which the 2894 processing following the end of mail data indication is only 2895 partially successful. This could happen if, after accepting several 2896 recipients and the mail data, the SMTP server finds that the mail 2897 data could be successfully delivered to some, but not all, of the 2898 recipients. In such cases, the response to the DATA command MUST be 2899 an OK reply. However, the SMTP server MUST compose and send an 2900 "undeliverable mail" notification message to the originator of the 2901 message. 2903 A single notification listing all of the failed recipients or 2904 separate notification messages MUST be sent for each failed 2905 recipient. For economy of processing by the sender, the former 2906 SHOULD be used when possible. Note that the key difference between 2907 handling aliases (Section 3.9.1) and forwarding (this subsection) is 2908 the change to the backward-pointing address in this case. All 2909 notification messages about undeliverable mail MUST be sent using the 2910 MAIL command (even if they result from processing the obsolete SEND, 2911 SOML, or SAML commands) and MUST use a null return path as discussed 2912 in Section 3.6. 2914 The time stamp line and the return path line are formally defined as 2915 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2916 [11]): 2918 Return-path-line = "Return-Path:" FWS Reverse-path 2920 Time-stamp-line = "Received:" FWS Stamp 2922 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2923 FWS date-time 2924 ; where "date-time" is as defined in RFC 5322 [11] 2925 ; but the "obs-" forms, especially two-digit 2926 ; years, are prohibited in SMTP and MUST NOT be used. 2928 From-domain = "FROM" FWS Extended-Domain 2930 By-domain = CFWS "BY" FWS Extended-Domain 2932 Extended-Domain = Domain / 2933 ( Domain FWS "(" TCP-info ")" ) / 2934 ( address-literal FWS "(" TCP-info ")" ) 2936 TCP-info = address-literal / ( Domain FWS address-literal ) 2937 ; Information derived by server from TCP connection 2938 ; not client EHLO. 2940 Opt-info = [Via] [With] [ID] [For] 2941 [Additional-Registered-Clauses] 2943 Via = CFWS "VIA" FWS Link 2945 With = CFWS "WITH" FWS Protocol 2946 ID = CFWS "ID" FWS ( Atom / msg-id ) 2947 ; msg-id is defined in RFC 5322 [11] 2949 For = CFWS "FOR" FWS ( Path / Mailbox ) 2951 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 2952 [[CREF24: [5321bis] 5321 errata #1683, 20090215, 2953 Roberto Javier Godoy, rjgodoy@fich.unl.edu.ar]] 2954 ; Additional standard clauses may be added in this 2955 ; location by future standards and registration with 2956 ; IANA. SMTP servers SHOULD NOT use unregistered 2957 ; names. See Section 8. 2959 Link = "TCP" / Addtl-Link 2961 Addtl-Link = Atom 2962 ; Additional standard names for links are 2963 ; registered with the Internet Assigned Numbers 2964 ; Authority (IANA). "Via" is primarily of value 2965 ; with non-Internet transports. SMTP servers 2966 ; SHOULD NOT use unregistered names. 2968 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 2970 Addtl-Protocol = Atom 2971 ; Additional standard names for protocols are 2972 ; registered with the Internet Assigned Numbers 2973 ; Authority (IANA) in the "mail parameters" 2974 ; registry [7]. SMTP servers SHOULD NOT 2975 ; use unregistered names. 2977 4.5. Additional Implementation Issues 2979 4.5.1. Minimum Implementation 2981 In order to make SMTP workable, the following minimum implementation 2982 MUST be provided by all receivers. The following commands MUST be 2983 supported to conform to this specification: 2985 EHLO 2986 HELO 2987 MAIL 2988 RCPT 2989 DATA 2990 RSET 2991 NOOP 2992 QUIT 2993 VRFY 2995 Any system that includes an SMTP server supporting mail relaying or 2996 delivery MUST support the reserved mailbox "postmaster" as a case- 2997 insensitive local name. This postmaster address is not strictly 2998 necessary if the server always returns 554 on connection opening (as 2999 described in Section 3.1). The requirement to accept mail for 3000 postmaster implies that RCPT commands that specify a mailbox for 3001 postmaster at any of the domains for which the SMTP server provides 3002 mail service, as well as the special case of "RCPT TO:" 3003 (with no domain specification), MUST be supported. 3005 SMTP systems are expected to make every reasonable effort to accept 3006 mail directed to Postmaster from any other system on the Internet. 3007 In extreme cases -- such as to contain a denial of service attack or 3008 other breach of security -- an SMTP server may block mail directed to 3009 Postmaster. However, such arrangements SHOULD be narrowly tailored 3010 so as to avoid blocking messages that are not part of such attacks. 3012 4.5.2. Transparency 3014 Without some provision for data transparency, the character sequence 3015 "." ends the mail text and cannot be sent by the user. 3016 In general, users are not aware of such "forbidden" sequences. To 3017 allow all user composed text to be transmitted transparently, the 3018 following procedures are used: 3020 o Before sending a line of mail text, the SMTP client checks the 3021 first character of the line. If it is a period, one additional 3022 period is inserted at the beginning of the line. 3024 o When a line of mail text is received by the SMTP server, it checks 3025 the line. If the line is composed of a single period, it is 3026 treated as the end of mail indicator. If the first character is a 3027 period and there are other characters on the line, the first 3028 character is deleted. 3030 The mail data may contain any of the 128 ASCII characters. All 3031 characters are to be delivered to the recipient's mailbox, including 3032 spaces, vertical and horizontal tabs, and other control characters. 3033 If the transmission channel provides an 8-bit byte (octet) data 3034 stream, the 7-bit ASCII codes are transmitted, right justified, in 3035 the octets, with the high-order bits cleared to zero. See 3036 Section 3.6 for special treatment of these conditions in SMTP systems 3037 serving a relay function. 3039 In some systems, it may be necessary to transform the data as it is 3040 received and stored. This may be necessary for hosts that use a 3041 different character set than ASCII as their local character set, that 3042 store data in records rather than strings, or which use special 3043 character sequences as delimiters inside mailboxes. If such 3044 transformations are necessary, they MUST be reversible, especially if 3045 they are applied to mail being relayed. 3047 4.5.3. Sizes and Timeouts 3049 4.5.3.1. Size Limits and Minimums 3051 There are several objects that have required minimum/maximum sizes. 3052 Every implementation MUST be able to receive objects of at least 3053 these sizes. Objects larger than these sizes SHOULD be avoided when 3054 possible. However, some Internet mail constructs such as encoded 3055 X.400 addresses (RFC 2156 [26]) will often require larger objects. 3056 Clients MAY attempt to transmit these, but MUST be prepared for a 3057 server to reject them if they cannot be handled by it. To the 3058 maximum extent possible, implementation techniques that impose no 3059 limits on the length of these objects should be used. 3061 Extensions to SMTP may involve the use of characters that occupy more 3062 than a single octet each. This section therefore specifies lengths 3063 in octets where absolute lengths, rather than character counts, are 3064 intended. 3066 [[CREF25: [5321bis] [[Note in Draft: Klensin 20191126: Given the 3067 controversy on the SMTP mailing list between 20191123 and now about 3068 maximum lengths, is the above adequate or is further tuning of the 3069 limit text below needed? ]]]] 3071 4.5.3.1.1. Local-part 3073 The maximum total length of a user name or other local-part is 64 3074 octets. 3076 4.5.3.1.2. Domain 3078 The maximum total length of a domain name or number is 255 octets. 3080 4.5.3.1.3. Path 3082 The maximum total length of a reverse-path or forward-path is 256 3083 octets (including the punctuation and element separators). 3085 4.5.3.1.4. Command Line 3087 The maximum total length of a command line including the command word 3088 and the is 512 octets. SMTP extensions may be used to 3089 increase this limit. 3091 4.5.3.1.5. Reply Line 3093 The maximum total length of a reply line including the reply code and 3094 the is 512 octets. More information may be conveyed through 3095 multiple-line replies. 3097 4.5.3.1.6. Text Line 3099 The maximum total length of a text line including the is 1000 3100 octets (not counting the leading dot duplicated for transparency). 3101 This number may be increased by the use of SMTP Service Extensions. 3103 4.5.3.1.7. Message Content 3105 The maximum total length of a message content (including any message 3106 header section as well as the message body) MUST BE at least 64K 3107 octets. Since the introduction of Internet Standards for multimedia 3108 mail (RFC 2045 [24]), message lengths on the Internet have grown 3109 dramatically, and message size restrictions should be avoided if at 3110 all possible. SMTP server systems that must impose restrictions 3111 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3112 SMTP client systems that will send large messages SHOULD utilize it 3113 when possible. 3115 4.5.3.1.8. Recipient Buffer 3117 The minimum total number of recipients that MUST be buffered is 100 3118 recipients. Rejection of messages (for excessive recipients) with 3119 fewer than 100 RCPT commands is a violation of this specification. 3120 The general principle that relaying SMTP server MUST NOT, and 3121 delivery SMTP servers SHOULD NOT, perform validation tests on message 3122 header fields suggests that messages SHOULD NOT be rejected based on 3123 the total number of recipients shown in header fields. A server that 3124 imposes a limit on the number of recipients MUST behave in an orderly 3125 fashion, such as rejecting additional addresses over its limit rather 3126 than silently discarding addresses previously accepted. A client 3127 that needs to deliver a message containing over 100 RCPT commands 3128 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3129 server declines to accept more than 100 recipients in a single 3130 message. 3132 4.5.3.1.9. Treatment When Limits Exceeded 3134 Errors due to exceeding these limits may be reported by using the 3135 reply codes. Some examples of reply codes are: 3137 500 Line too long. 3139 or 3141 501 Path too long 3143 or 3145 452 Too many recipients (see below) 3147 or 3149 552 Too much mail data. 3151 4.5.3.1.10. Too Many Recipients Code 3153 RFC 821 [3] incorrectly listed the error where an SMTP server 3154 exhausts its implementation limit on the number of RCPT commands 3155 ("too many recipients") as having reply code 552. The correct reply 3156 code for this condition is 452. Clients SHOULD treat a 552 code in 3157 this case as a temporary, rather than permanent, failure so the logic 3158 below works. 3160 When a conforming SMTP server encounters this condition, it has at 3161 least 100 successful RCPT commands in its recipient buffer. If the 3162 server is able to accept the message, then at least these 100 3163 addresses will be removed from the SMTP client's queue. When the 3164 client attempts retransmission of those addresses that received 452 3165 responses, at least 100 of these will be able to fit in the SMTP 3166 server's recipient buffer. Each retransmission attempt that is able 3167 to deliver anything will be able to dispose of at least 100 of these 3168 recipients. 3170 If an SMTP server has an implementation limit on the number of RCPT 3171 commands and this limit is exhausted, it MUST use a response code of 3172 452 (but the client SHOULD also be prepared for a 552, as noted 3173 above). If the server has a configured site-policy limitation on the 3174 number of RCPT commands, it MAY instead use a 5yz response code. In 3175 particular, if the intent is to prohibit messages with more than a 3176 site-specified number of recipients, rather than merely limit the 3177 number of recipients in a given mail transaction, it would be 3178 reasonable to return a 503 response to any DATA command received 3179 subsequent to the 452 (or 552) code or to simply return the 503 after 3180 DATA without returning any previous negative response. 3182 4.5.3.2. Timeouts 3184 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3185 command timeouts rather than somehow trying to time the entire mail 3186 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3187 without recompiling the SMTP code. To implement this, a timer is set 3188 for each SMTP command and for each buffer of the data transfer. The 3189 latter means that the overall timeout is inherently proportional to 3190 the size of the message. 3192 Based on extensive experience with busy mail-relay hosts, the minimum 3193 per-command timeout values SHOULD be as follows: 3195 4.5.3.2.1. Initial 220 Message: 5 Minutes 3197 An SMTP client process needs to distinguish between a failed TCP 3198 connection and a delay in receiving the initial 220 greeting message. 3199 Many SMTP servers accept a TCP connection but delay delivery of the 3200 220 message until their system load permits more mail to be 3201 processed. 3203 4.5.3.2.2. MAIL Command: 5 Minutes 3205 4.5.3.2.3. RCPT Command: 5 Minutes 3207 A longer timeout is required if processing of mailing lists and 3208 aliases is not deferred until after the message was accepted. 3210 4.5.3.2.4. DATA Initiation: 2 Minutes 3212 This is while awaiting the "354 Start Input" reply to a DATA command. 3214 4.5.3.2.5. Data Block: 3 Minutes 3216 This is while awaiting the completion of each TCP SEND call 3217 transmitting a chunk of data. 3219 4.5.3.2.6. DATA Termination: 10 Minutes. 3221 This is while awaiting the "250 OK" reply. When the receiver gets 3222 the final period terminating the message data, it typically performs 3223 processing to deliver the message to a user mailbox. A spurious 3224 timeout at this point would be very wasteful and would typically 3225 result in delivery of multiple copies of the message, since it has 3226 been successfully sent and the server has accepted responsibility for 3227 delivery. See Section 6.1 for additional discussion. 3229 4.5.3.2.7. Server Timeout: 5 Minutes. 3231 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3232 is awaiting the next command from the sender. 3234 4.5.4. Retry Strategies 3236 The common structure of a host SMTP implementation includes user 3237 mailboxes, one or more areas for queuing messages in transit, and one 3238 or more daemon processes for sending and receiving mail. The exact 3239 structure will vary depending on the needs of the users on the host 3240 and the number and size of mailing lists supported by the host. We 3241 describe several optimizations that have proved helpful, particularly 3242 for mailers supporting high traffic levels. 3244 Any queuing strategy MUST include timeouts on all activities on a 3245 per-command basis. A queuing strategy MUST NOT send error messages 3246 in response to error messages under any circumstances. 3248 4.5.4.1. Sending Strategy 3250 The general model for an SMTP client is one or more processes that 3251 periodically attempt to transmit outgoing mail. In a typical system, 3252 the program that composes a message has some method for requesting 3253 immediate attention for a new piece of outgoing mail, while mail that 3254 cannot be transmitted immediately MUST be queued and periodically 3255 retried by the sender. A mail queue entry will include not only the 3256 message itself but also the envelope information. 3258 The sender MUST delay retrying a particular destination after one 3259 attempt has failed. In general, the retry interval SHOULD be at 3260 least 30 minutes; however, more sophisticated and variable strategies 3261 will be beneficial when the SMTP client can determine the reason for 3262 non-delivery. 3264 Retries continue until the message is transmitted or the sender gives 3265 up; the give-up time generally needs to be at least 4-5 days. It MAY 3266 be appropriate to set a shorter maximum number of retries for non- 3267 delivery notifications and equivalent error messages than for 3268 standard messages. The parameters to the retry algorithm MUST be 3269 configurable. 3271 A client SHOULD keep a list of hosts it cannot reach and 3272 corresponding connection timeouts, rather than just retrying queued 3273 mail items. 3275 Experience suggests that failures are typically transient (the target 3276 system or its connection has crashed), favoring a policy of two 3277 connection attempts in the first hour the message is in the queue, 3278 and then backing off to one every two or three hours. 3280 The SMTP client can shorten the queuing delay in cooperation with the 3281 SMTP server. For example, if mail is received from a particular 3282 address, it is likely that mail queued for that host can now be sent. 3283 Application of this principle may, in many cases, eliminate the 3284 requirement for an explicit "send queues now" function such as ETRN, 3285 RFC 1985 [23]. 3287 The strategy may be further modified as a result of multiple 3288 addresses per host (see below) to optimize delivery time versus 3289 resource usage. 3291 An SMTP client may have a large queue of messages for each 3292 unavailable destination host. If all of these messages were retried 3293 in every retry cycle, there would be excessive Internet overhead and 3294 the sending system would be blocked for a long period. Note that an 3295 SMTP client can generally determine that a delivery attempt has 3296 failed only after a timeout of several minutes, and even a one-minute 3297 timeout per connection will result in a very large delay if retries 3298 are repeated for dozens, or even hundreds, of queued messages to the 3299 same host. 3301 At the same time, SMTP clients SHOULD use great care in caching 3302 negative responses from servers. In an extreme case, if EHLO is 3303 issued multiple times during the same SMTP connection, different 3304 answers may be returned by the server. More significantly, 5yz 3305 responses to the MAIL command MUST NOT be cached. 3307 When a mail message is to be delivered to multiple recipients, and 3308 the SMTP server to which a copy of the message is to be sent is the 3309 same for multiple recipients, then only one copy of the message 3310 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3311 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3312 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3313 are very many addresses, a limit on the number of RCPT commands per 3314 MAIL command MAY be imposed. This efficiency feature SHOULD be 3315 implemented. 3317 Similarly, to achieve timely delivery, the SMTP client MAY support 3318 multiple concurrent outgoing mail transactions. However, some limit 3319 may be appropriate to protect the host from devoting all its 3320 resources to mail. 3322 4.5.4.2. Receiving Strategy 3324 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3325 port (specified by IANA as port 25) at all times. This requires the 3326 support of multiple incoming TCP connections for SMTP. Some limit 3327 MAY be imposed, but servers that cannot handle more than one SMTP 3328 transaction at a time are not in conformance with the intent of this 3329 specification. 3331 As discussed above, when the SMTP server receives mail from a 3332 particular host address, it could activate its own SMTP queuing 3333 mechanisms to retry any mail pending for that host address. 3335 4.5.5. Messages with a Null Reverse-Path 3337 There are several types of notification messages that are required by 3338 existing and proposed Standards to be sent with a null reverse-path, 3339 namely non-delivery notifications as discussed in Section 3.7, other 3340 kinds of Delivery Status Notifications (DSNs, RFC 3461 [33]), and 3341 Message Disposition Notifications (MDNs, RFC 3798 [37]). All of 3342 these kinds of messages are notifications about a previous message, 3343 and they are sent to the reverse-path of the previous mail message. 3344 (If the delivery of such a notification message fails, that usually 3345 indicates a problem with the mail system of the host to which the 3346 notification message is addressed. For this reason, at some hosts 3347 the MTA is set up to forward such failed notification messages to 3348 someone who is able to fix problems with the mail system, e.g., via 3349 the postmaster alias.) 3351 All other types of messages (i.e., any message which is not required 3352 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3353 with a valid, non-null reverse-path. 3355 Implementers of automated email processors should be careful to make 3356 sure that the various kinds of messages with a null reverse-path are 3357 handled correctly. In particular, such systems SHOULD NOT reply to 3358 messages with a null reverse-path, and they SHOULD NOT add a non-null 3359 reverse-path, or change a null reverse-path to a non-null one, to 3360 such messages when forwarding. 3362 5. Address Resolution and Mail Handling 3364 5.1. Locating the Target Host 3366 Once an SMTP client lexically identifies a domain to which mail will 3367 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3368 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3369 [4]). The names are expected to be fully-qualified domain names 3370 (FQDNs): mechanisms for inferring FQDNs from partial names or local 3371 aliases are outside of this specification. Due to a history of 3372 problems, SMTP servers used for initial submission of messages SHOULD 3373 NOT make such inferences (Message Submission Servers [42] have 3374 somewhat more flexibility) and intermediate (relay) SMTP servers MUST 3375 NOT make them. 3377 The lookup first attempts to locate an MX record associated with the 3378 name. If a CNAME record is found, the resulting name is processed as 3379 if it were the initial name. If a non-existent domain error is 3380 returned, this situation MUST be reported as an error. If a 3381 temporary error is returned, the message MUST be queued and retried 3382 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3383 the address is treated as if it was associated with an implicit MX 3384 RR, with a preference of 0, pointing to that host. If MX records are 3385 present, but none of them are usable, or the implicit MX is unusable, 3386 this situation MUST be reported as an error. 3388 If one or more MX RRs are found for a given name, SMTP systems MUST 3389 NOT utilize any address RRs associated with that name unless they are 3390 located using the MX RRs; the "implicit MX" rule above applies only 3391 if there are no MX records present. If MX records are present, but 3392 none of them are usable, this situation MUST be reported as an error. 3394 When a domain name associated with an MX RR is looked up and the 3395 associated data field obtained, the data field of that response MUST 3396 contain a domain name that conforms to the specifications of 3397 Section 2.3.5. 3398 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3399 unfolds, some additional text may be in order here (20140718)]] 3400 That domain name, when queried, MUST return at least one address 3401 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3402 server to which the message should be directed. Any other response, 3403 specifically including a value that will return a CNAME record when 3404 queried, lies outside the scope of this Standard. The prohibition on 3405 labels in the data that resolve to CNAMEs is discussed in more detail 3406 in RFC 2181, Section 10.3 [27]. 3408 When the lookup succeeds, the mapping can result in a list of 3409 alternative delivery addresses rather than a single address, because 3410 of multiple MX records, multihoming, or both. To provide reliable 3411 mail transmission, the SMTP client MUST be able to try (and retry) 3412 each of the relevant addresses in this list in order, until a 3413 delivery attempt succeeds. However, there MAY also be a configurable 3414 limit on the number of alternate addresses that can be tried. In any 3415 case, the SMTP client SHOULD try at least two addresses. 3417 Two types of information are used to rank the host addresses: 3418 multiple MX records, and multihomed hosts. 3420 MX records contain a preference indication that MUST be used in 3421 sorting if more than one such record appears (see below). Lower 3422 numbers are more preferred than higher ones. If there are multiple 3423 destinations with the same preference and there is no clear reason to 3424 favor one (e.g., by recognition of an easily reached address), then 3425 the sender-SMTP MUST randomize them to spread the load across 3426 multiple mail exchangers for a specific organization. 3428 The destination host (perhaps taken from the preferred MX record) may 3429 be multihomed, in which case the domain name resolver will return a 3430 list of alternative IP addresses. It is the responsibility of the 3431 domain name resolver interface to have ordered this list by 3432 decreasing preference if necessary, and the SMTP sender MUST try them 3433 in the order presented. 3435 Although the capability to try multiple alternative addresses is 3436 required, specific installations may want to limit or disable the use 3437 of alternative addresses. The question of whether a sender should 3438 attempt retries using the different addresses of a multihomed host 3439 has been controversial. The main argument for using the multiple 3440 addresses is that it maximizes the probability of timely delivery, 3441 and indeed sometimes the probability of any delivery; the counter- 3442 argument is that it may result in unnecessary resource use. Note 3443 that resource use is also strongly determined by the sending strategy 3444 discussed in Section 4.5.4.1. 3446 If an SMTP server receives a message with a destination for which it 3447 is a designated Mail eXchanger, it MAY relay the message (potentially 3448 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3449 final delivery of the message, or hand it off using some mechanism 3450 outside the SMTP-provided transport environment. Of course, neither 3451 of the latter require that the list of MX records be examined 3452 further. 3454 If it determines that it should relay the message without rewriting 3455 the address, it MUST sort the MX records to determine candidates for 3456 delivery. The records are first ordered by preference, with the 3457 lowest-numbered records being most preferred. The relay host MUST 3458 then inspect the list for any of the names or addresses by which it 3459 might be known in mail transactions. If a matching record is found, 3460 all records at that preference level and higher-numbered ones MUST be 3461 discarded from consideration. If there are no records left at that 3462 point, it is an error condition, and the message MUST be returned as 3463 undeliverable. If records do remain, they SHOULD be tried, best 3464 preference first, as described above. 3466 5.2. IPv6 and MX Records 3468 In the contemporary Internet, SMTP clients and servers may be hosted 3469 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3470 compatible with either version of the Internet Protocol. The host 3471 domains to which MX records point may, consequently, contain "A RR"s 3472 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3473 3974 [39] discusses some operational experience in mixed 3474 environments, it was not comprehensive enough to justify 3475 standardization, and some of its recommendations appear to be 3476 inconsistent with this specification. The appropriate actions to be 3477 taken either will depend on local circumstances, such as performance 3478 of the relevant networks and any conversions that might be necessary, 3479 or will be obvious (e.g., an IPv6-only client need not attempt to 3480 look up A RRs or attempt to reach IPv4-only servers). Designers of 3481 SMTP implementations that might run in IPv6 or dual-stack 3482 environments should study the procedures above, especially the 3483 comments about multihomed hosts, and, preferably, provide mechanisms 3484 to facilitate operational tuning and mail interoperability between 3485 IPv4 and IPv6 systems while considering local circumstances. 3487 6. Problem Detection and Handling 3489 6.1. Reliable Delivery and Replies by Email 3491 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3492 message in response to DATA), it is accepting responsibility for 3493 delivering or relaying the message. It must take this responsibility 3494 seriously. It MUST NOT lose the message for frivolous reasons, such 3495 as because the host later crashes or because of a predictable 3496 resource shortage. Some reasons that are not considered frivolous 3497 are discussed in the next subsection and in Section 7.8. 3499 If there is a delivery failure after acceptance of a message, the 3500 receiver-SMTP MUST formulate and mail a notification message. This 3501 notification MUST be sent using a null ("<>") reverse-path in the 3502 envelope. The recipient of this notification MUST be the address 3503 from the envelope return path (or the Return-Path: line). However, 3504 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3505 notification. Obviously, nothing in this section can or should 3506 prohibit local decisions (i.e., as part of the same system 3507 environment as the receiver-SMTP) to log or otherwise transmit 3508 information about null address events locally if that is desired. If 3509 the address is an explicit source route, it MUST be stripped down to 3510 its final hop. 3512 For example, suppose that an error notification must be sent for a 3513 message that arrived with: 3515 MAIL FROM:<@a,@b:user@d> 3517 The notification message MUST be sent using: 3519 RCPT TO: 3521 Some delivery failures after the message is accepted by SMTP will be 3522 unavoidable. For example, it may be impossible for the receiving 3523 SMTP server to validate all the delivery addresses in RCPT command(s) 3524 due to a "soft" domain system error, because the target is a mailing 3525 list (see earlier discussion of RCPT), or because the server is 3526 acting as a relay and has no immediate access to the delivering 3527 system. 3529 To avoid receiving duplicate messages as the result of timeouts, a 3530 receiver-SMTP MUST seek to minimize the time required to respond to 3531 the final . end of data indicator. See RFC 1047 [16] for 3532 a discussion of this problem. 3534 6.2. Unwanted, Unsolicited, and "Attack" Messages 3536 Utility and predictability of the Internet mail system requires that 3537 messages that can be delivered should be delivered, regardless of any 3538 syntax or other faults associated with those messages and regardless 3539 of their content. If they cannot be delivered, and cannot be 3540 rejected by the SMTP server during the SMTP transaction, they should 3541 be "bounced" (returned with non-delivery notification messages) as 3542 described above. In today's world, in which many SMTP server 3543 operators have discovered that the quantity of undesirable bulk email 3544 vastly exceeds the quantity of desired mail and in which accepting a 3545 message may trigger additional undesirable traffic by providing 3546 verification of the address, those principles may not be practical. 3548 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3549 without notification of the sender is permitted in practice. 3550 However, it is extremely dangerous and violates a long tradition and 3551 community expectations that mail is either delivered or returned. If 3552 silent message-dropping is misused, it could easily undermine 3553 confidence in the reliability of the Internet's mail systems. So 3554 silent dropping of messages should be considered only in those cases 3555 where there is very high confidence that the messages are seriously 3556 fraudulent or otherwise inappropriate. 3558 To stretch the principle of delivery if possible even further, it may 3559 be a rational policy to not deliver mail that has an invalid return 3560 address, although the history of the network is that users are 3561 typically better served by delivering any message that can be 3562 delivered. Reliably determining that a return address is invalid can 3563 be a difficult and time-consuming process, especially if the putative 3564 sending system is not directly accessible or does not fully and 3565 accurately support VRFY and, even if a "drop messages with invalid 3566 return addresses" policy is adopted, it SHOULD be applied only when 3567 there is near-certainty that the return addresses are, in fact, 3568 invalid. 3570 Conversely, if a message is rejected because it is found to contain 3571 hostile content (a decision that is outside the scope of an SMTP 3572 server as defined in this document), rejection ("bounce") messages 3573 SHOULD NOT be sent unless the receiving site is confident that those 3574 messages will be usefully delivered. The preference and default in 3575 these cases is to avoid sending non-delivery messages when the 3576 incoming message is determined to contain hostile content. 3578 6.3. Loop Detection 3580 Simple counting of the number of "Received:" header fields in a 3581 message has proven to be an effective, although rarely optimal, 3582 method of detecting loops in mail systems. SMTP servers using this 3583 technique SHOULD use a large rejection threshold, normally at least 3584 100 Received entries. Whatever mechanisms are used, servers MUST 3585 contain provisions for detecting and stopping trivial loops. 3587 6.4. Compensating for Irregularities 3589 Unfortunately, variations, creative interpretations, and outright 3590 violations of Internet mail protocols do occur; some would suggest 3591 that they occur quite frequently. The debate as to whether a well- 3592 behaved SMTP receiver or relay should reject a malformed message, 3593 attempt to pass it on unchanged, or attempt to repair it to increase 3594 the odds of successful delivery (or subsequent reply) began almost 3595 with the dawn of structured network mail and shows no signs of 3596 abating. Advocates of rejection claim that attempted repairs are 3597 rarely completely adequate and that rejection of bad messages is the 3598 only way to get the offending software repaired. Advocates of 3599 "repair" or "deliver no matter what" argue that users prefer that 3600 mail go through it if at all possible and that there are significant 3601 market pressures in that direction. In practice, these market 3602 pressures may be more important to particular vendors than strict 3603 conformance to the standards, regardless of the preference of the 3604 actual developers. 3606 The problems associated with ill-formed messages were exacerbated by 3607 the introduction of the split-UA mail reading protocols (Post Office 3608 Protocol (POP) version 2 [13], Post Office Protocol (POP) version 3 3609 [22], IMAP version 2 [18], and PCMAIL [17]). These protocols 3610 encouraged the use of SMTP as a posting (message submission) 3611 protocol, and SMTP servers as relay systems for these client hosts 3612 (which are often only intermittently connected to the Internet). 3613 Historically, many of those client machines lacked some of the 3614 mechanisms and information assumed by SMTP (and indeed, by the mail 3615 format protocol, RFC 822 [12]). Some could not keep adequate track 3616 of time; others had no concept of time zones; still others could not 3617 identify their own names or addresses; and, of course, none could 3618 satisfy the assumptions that underlay RFC 822's conception of 3619 authenticated addresses. 3621 In response to these weak SMTP clients, many SMTP systems now 3622 complete messages that are delivered to them in incomplete or 3623 incorrect form. This strategy is generally considered appropriate 3624 when the server can identify or authenticate the client, and there 3625 are prior agreements between them. By contrast, there is at best 3626 great concern about fixes applied by a relay or delivery SMTP server 3627 that has little or no knowledge of the user or client machine. Many 3628 of these issues are addressed by using a separate protocol, such as 3629 that defined in RFC 4409 [42], for message submission, rather than 3630 using originating SMTP servers for that purpose. 3632 The following changes to a message being processed MAY be applied 3633 when necessary by an originating SMTP server, or one used as the 3634 target of SMTP as an initial posting (message submission) protocol: 3636 o Addition of a message-id field when none appears 3638 o Addition of a date, time, or time zone when none appears 3640 o Correction of addresses to proper FQDN format 3642 The less information the server has about the client, the less likely 3643 these changes are to be correct and the more caution and conservatism 3644 should be applied when considering whether or not to perform fixes 3645 and how. These changes MUST NOT be applied by an SMTP server that 3646 provides an intermediate relay function. 3648 In all cases, properly operating clients supplying correct 3649 information are preferred to corrections by the SMTP server. In all 3650 cases, documentation SHOULD be provided in trace header fields and/or 3651 header field comments for actions performed by the servers. 3653 7. Security Considerations 3655 7.1. Mail Security and Spoofing 3657 SMTP mail is inherently insecure in that it is feasible for even 3658 fairly casual users to negotiate directly with receiving and relaying 3659 SMTP servers and create messages that will trick a naive recipient 3660 into believing that they came from somewhere else. Constructing such 3661 a message so that the "spoofed" behavior cannot be detected by an 3662 expert is somewhat more difficult, but not sufficiently so as to be a 3663 deterrent to someone who is determined and knowledgeable. 3664 Consequently, as knowledge of Internet mail increases, so does the 3665 knowledge that SMTP mail inherently cannot be authenticated, or 3666 integrity checks provided, at the transport level. Real mail 3667 security lies only in end-to-end methods involving the message 3668 bodies, such as those that use digital signatures (see RFC 1847 [20] 3669 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [45] or Secure/ 3670 Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [38]). 3672 Various protocol extensions and configuration options that provide 3673 authentication at the transport level (e.g., from an SMTP client to 3674 an SMTP server) improve somewhat on the traditional situation 3675 described above. However, in general, they only authenticate one 3676 server to another rather than a chain of relays and servers, much 3677 less authenticating users or user machines. Consequently, unless 3678 they are accompanied by careful handoffs of responsibility in a 3679 carefully designed trust environment, they remain inherently weaker 3680 than end-to-end mechanisms that use digitally signed messages rather 3681 than depending on the integrity of the transport system. 3683 Efforts to make it more difficult for users to set envelope return 3684 path and header "From" fields to point to valid addresses other than 3685 their own are largely misguided: they frustrate legitimate 3686 applications in which mail is sent by one user on behalf of another, 3687 in which error (or normal) replies should be directed to a special 3688 address, or in which a single message is sent to multiple recipients 3689 on different hosts. (Systems that provide convenient ways for users 3690 to alter these header fields on a per-message basis should attempt to 3691 establish a primary and permanent mailbox address for the user so 3692 that Sender header fields within the message data can be generated 3693 sensibly.) 3695 This specification does not further address the authentication issues 3696 associated with SMTP other than to advocate that useful functionality 3697 not be disabled in the hope of providing some small margin of 3698 protection against a user who is trying to fake mail. 3700 7.2. "Blind" Copies 3702 Addresses that do not appear in the message header section may appear 3703 in the RCPT commands to an SMTP server for a number of reasons. The 3704 two most common involve the use of a mailing address as a "list 3705 exploder" (a single address that resolves into multiple addresses) 3706 and the appearance of "blind copies". Especially when more than one 3707 RCPT command is present, and in order to avoid defeating some of the 3708 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3709 the full set of RCPT command arguments into the header section, 3710 either as part of trace header fields or as informational or private- 3711 extension header fields. [[CREF26: [rfc5321bis] [[Note in draft - 3712 Suggestion from 20070124 that got lost: delete "especially" and "the 3713 full set of" -- copying the first one can be as harmful as copying 3714 all of them, at least without verifying that the addresses do appear 3715 in the headers.]] Arnt Gulbrandsen, arnt@oryx.com, 2007.01.24 3716 1121+0100]] Since this rule is often violated in practice, and cannot 3717 be enforced, sending SMTP systems that are aware of "bcc" use MAY 3718 find it helpful to send each blind copy as a separate message 3719 transaction containing only a single RCPT command. 3721 There is no inherent relationship between either "reverse" (from 3722 MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP 3723 transaction ("envelope") and the addresses in the header section. 3724 Receiving systems SHOULD NOT attempt to deduce such relationships and 3725 use them to alter the header section of the message for delivery. 3726 The popular "Apparently-to" header field is a violation of this 3727 principle as well as a common source of unintended information 3728 disclosure and SHOULD NOT be used. 3730 7.3. VRFY, EXPN, and Security 3732 As discussed in Section 3.5, individual sites may want to disable 3733 either or both of VRFY or EXPN for security reasons (see below). As 3734 a corollary to the above, implementations that permit this MUST NOT 3735 appear to have verified addresses that are not, in fact, verified. 3736 If a site disables these commands for security reasons, the SMTP 3737 server MUST return a 252 response, rather than a code that could be 3738 confused with successful or unsuccessful verification. 3740 Returning a 250 reply code with the address listed in the VRFY 3741 command after having checked it only for syntax violates this rule. 3742 Of course, an implementation that "supports" VRFY by always returning 3743 550 whether or not the address is valid is equally not in 3744 conformance. 3746 On the public Internet, the contents of mailing lists have become 3747 popular as an address information source for so-called "spammers." 3748 The use of EXPN to "harvest" addresses has increased as list 3749 administrators have installed protections against inappropriate uses 3750 of the lists themselves. However, VRFY and EXPN are still useful for 3751 authenticated users and within an administrative domain. For 3752 example, VRFY and EXPN are useful for performing internal audits of 3753 how email gets routed to check and to make sure no one is 3754 automatically forwarding sensitive mail outside the organization. 3755 Sites implementing SMTP authentication may choose to make VRFY and 3756 EXPN available only to authenticated requestors. Implementations 3757 SHOULD still provide support for EXPN, but sites SHOULD carefully 3758 evaluate the tradeoffs. 3760 Whether disabling VRFY provides any real marginal security depends on 3761 a series of other conditions. In many cases, RCPT commands can be 3762 used to obtain the same information about address validity. On the 3763 other hand, especially in situations where determination of address 3764 validity for RCPT commands is deferred until after the DATA command 3765 is received, RCPT may return no information at all, while VRFY is 3766 expected to make a serious attempt to determine validity before 3767 generating a response code (see discussion above). 3769 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3771 Before a client uses the 251 or 551 reply codes from a RCPT command 3772 to automatically update its future behavior (e.g., updating the 3773 user's address book), it should be certain of the server's 3774 authenticity. If it does not, it may be subject to a man in the 3775 middle attack. 3777 7.5. Information Disclosure in Announcements 3779 There has been an ongoing debate about the tradeoffs between the 3780 debugging advantages of announcing server type and version (and, 3781 sometimes, even server domain name) in the greeting response or in 3782 response to the HELP command and the disadvantages of exposing 3783 information that might be useful in a potential hostile attack. The 3784 utility of the debugging information is beyond doubt. Those who 3785 argue for making it available point out that it is far better to 3786 actually secure an SMTP server rather than hope that trying to 3787 conceal known vulnerabilities by hiding the server's precise identity 3788 will provide more protection. Sites are encouraged to evaluate the 3789 tradeoff with that issue in mind; implementations SHOULD minimally 3790 provide for making type and version information available in some way 3791 to other network hosts. 3793 7.6. Information Disclosure in Trace Fields 3795 In some circumstances, such as when mail originates from within a LAN 3796 whose hosts are not directly on the public Internet, trace 3797 ("Received") header fields produced in conformance with this 3798 specification may disclose host names and similar information that 3799 would not normally be available. This ordinarily does not pose a 3800 problem, but sites with special concerns about name disclosure should 3801 be aware of it. Also, the optional FOR clause should be supplied 3802 with caution or not at all when multiple recipients are involved lest 3803 it inadvertently disclose the identities of "blind copy" recipients 3804 to others. 3806 7.7. Information Disclosure in Message Forwarding 3808 As discussed in Section 3.4, use of the 251 or 551 reply codes to 3809 identify the replacement address associated with a mailbox may 3810 inadvertently disclose sensitive information. Sites that are 3811 concerned about those issues should ensure that they select and 3812 configure servers appropriately. 3814 7.8. Resistance to Attacks 3816 In recent years, there has been an increase of attacks on SMTP 3817 servers, either in conjunction with attempts to discover addresses 3818 for sending unsolicited messages or simply to make the servers 3819 inaccessible to others (i.e., as an application-level denial of 3820 service attack). While the means of doing so are beyond the scope of 3821 this Standard, rational operational behavior requires that servers be 3822 permitted to detect such attacks and take action to defend 3823 themselves. For example, if a server determines that a large number 3824 of RCPT TO commands are being sent, most or all with invalid 3825 addresses, as part of such an attack, it would be reasonable for the 3826 server to close the connection after generating an appropriate number 3827 of 5yz (normally 550) replies. 3829 7.9. Scope of Operation of SMTP Servers 3831 It is a well-established principle that an SMTP server may refuse to 3832 accept mail for any operational or technical reason that makes sense 3833 to the site providing the server. However, cooperation among sites 3834 and installations makes the Internet possible. If sites take 3835 excessive advantage of the right to reject traffic, the ubiquity of 3836 email availability (one of the strengths of the Internet) will be 3837 threatened; considerable care should be taken and balance maintained 3838 if a site decides to be selective about the traffic it will accept 3839 and process. 3841 In recent years, use of the relay function through arbitrary sites 3842 has been used as part of hostile efforts to hide the actual origins 3843 of mail. Some sites have decided to limit the use of the relay 3844 function to known or identifiable sources, and implementations SHOULD 3845 provide the capability to perform this type of filtering. When mail 3846 is rejected for these or other policy reasons, a 550 code SHOULD be 3847 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3849 8. IANA Considerations 3851 IANA maintains three registries in support of this specification, all 3852 of which were created for RFC 2821 or earlier. This document expands 3853 the third one as specified below. The registry references listed are 3854 as of the time of publication; IANA does not guarantee the locations 3855 associated with the URLs. The registries are as follows: 3857 o The first, "Simple Mail Transfer Protocol (SMTP) Service 3858 Extensions" [49], consists of SMTP service extensions with the 3859 associated keywords, and, as needed, parameters and verbs. As 3860 specified in Section 2.2.2, no entry may be made in this registry 3861 that starts in an "X". Entries may be made only for service 3862 extensions (and associated keywords, parameters, or verbs) that 3863 are defined in Standards-Track or Experimental RFCs specifically 3864 approved by the IESG for this purpose. 3866 o The second registry, "Address Literal Tags" [50], consists of 3867 "tags" that identify forms of domain literals other than those for 3868 IPv4 addresses (specified in RFC 821 and in this document). The 3869 initial entry in that registry is for IPv6 addresses (specified in 3870 this document). Additional literal types require standardization 3871 before being used; none are anticipated at this time. 3873 o The third, "Mail Transmission Types" [49], established by RFC 821 3874 and renewed by this specification, is a registry of link and 3875 protocol identifiers to be used with the "via" and "with" 3876 subclauses of the time stamp ("Received:" header field) described 3877 in Section 4.4. Link and protocol identifiers in addition to 3878 those specified in this document may be registered only by 3879 standardization or by way of an RFC-documented, IESG-approved, 3880 Experimental protocol extension. This name space is for 3881 identification and not limited in size: the IESG is encouraged to 3882 approve on the basis of clear documentation and a distinct method 3883 rather than preferences about the properties of the method itself. 3885 An additional subsection has been added to the "VIA link types" 3886 and "WITH protocol types" subsections of this registry to contain 3887 registrations of "Additional-registered-clauses" as described 3888 above. The registry will contain clause names, a description, a 3889 summary of the syntax of the associated String, and a reference. 3890 As new clauses are defined, they may, in principle, specify 3891 creation of their own registries if the Strings consist of 3892 reserved terms or keywords rather than less restricted strings. 3893 As with link and protocol identifiers, additional clauses may be 3894 registered only by standardization or by way of an RFC-documented, 3895 IESG-approved, Experimental protocol extension. The additional 3896 clause name space is for identification and is not limited in 3897 size: the IESG is encouraged to approve on the basis of clear 3898 documentation, actual use or strong signs that the clause will be 3899 used, and a distinct requirement rather than preferences about the 3900 properties of the clause itself. 3902 In addition, if additional trace header fields (i.e., in addition to 3903 Return-path and Received) are ever created, those trace fields MUST 3904 be added to the IANA registry established by BCP 90 (RFC 3864) [8] 3905 for use with RFC 5322 [11]. 3907 9. Acknowledgments 3909 Many people contributed to the development of RFCs 2821 and 5321. 3910 Those documents should be consulted for those acknowledgments. 3912 Neither this document nor RFCs 2821 or 5321 would have been possible 3913 without the many contribution and insights of the late Jon Postel. 3914 Those contributions of course include the original specification of 3915 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3916 appears in this document as do several of Jon's original examples 3917 that have been updated only as needed to reflect other changes in the 3918 specification. 3920 The following filed errata against RFC 5321 that were not rejected at 3921 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3922 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3923 Rodrigo Speller, Alessandro Vesely, and Brett Watson. In addition, 3924 specific suggestions that led to corrections and improvements in this 3925 version were received from Ned Freed, Barry Leiba, Ivar Lumi, Pete 3926 Resnick, and others. 3928 chetti contributed an analysis that clarify the ABNF productions that 3929 implicitly reference other document. 3931 [[CREF27: Most errata and comments after 2019-07-01 have not yet been 3932 captured in this version of the draft. ]] 3934 10. References 3936 10.1. Normative References 3938 [1] Bradner, S., "Key words for use in RFCs to Indicate 3939 Requirement Levels", BCP 14, RFC 2119, 3940 DOI 10.17487/RFC2119, March 1997, 3941 . 3943 [2] American National Standards Institute (formerly United 3944 States of America Standards Institute), "USA Code for 3945 Information Interchange", ANSI X3.4-1968, 1968. 3947 ANSI X3.4-1968 has been replaced by newer versions with 3948 slight modifications, but the 1968 version remains 3949 definitive for the Internet. 3951 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 3952 RFC 821, DOI 10.17487/RFC0821, August 1982, 3953 . 3955 [4] Mockapetris, P., "Domain names - implementation and 3956 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3957 November 1987, . 3959 [5] Braden, R., Ed., "Requirements for Internet Hosts - 3960 Application and Support", STD 3, RFC 1123, 3961 DOI 10.17487/RFC1123, October 1989, 3962 . 3964 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 3965 Extension for Message Size Declaration", STD 10, RFC 1870, 3966 DOI 10.17487/RFC1870, November 1995, 3967 . 3969 [7] Newman, C., "ESMTP and LMTP Transmission Types 3970 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 3971 . 3973 [8] Klyne, G., Nottingham, M., and J. Mogul, "Registration 3974 Procedures for Message Header Fields", BCP 90, RFC 3864, 3975 DOI 10.17487/RFC3864, September 2004, 3976 . 3978 [9] Hinden, R. and S. Deering, "IP Version 6 Addressing 3979 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3980 2006, . 3982 [10] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 3983 Specifications: ABNF", STD 68, RFC 5234, 3984 DOI 10.17487/RFC5234, January 2008, 3985 . 3987 [11] Resnick, P., "Internet Message Format", RFC 5322, 3988 September 2008. 3990 10.2. Informative References 3992 [12] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 3993 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 3994 August 1982, . 3996 [13] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 3997 Reynolds, "Post Office Protocol: Version 2", RFC 937, 3998 DOI 10.17487/RFC0937, February 1985, 3999 . 4001 [14] Postel, J. and J. Reynolds, "File Transfer Protocol", 4002 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4003 . 4005 [15] Partridge, C., "Mail routing and the domain system", 4006 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4007 . 4009 [16] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4010 DOI 10.17487/RFC1047, February 1988, 4011 . 4013 [17] Lambert, M., "PCMAIL: A distributed mail system for 4014 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4015 1988, . 4017 [18] Crispin, M., "Interactive Mail Access Protocol: Version 4018 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4019 . 4021 [19] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4022 DOI 10.17487/RFC1846, September 1995, 4023 . 4025 [20] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4026 "Security Multiparts for MIME: Multipart/Signed and 4027 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4028 October 1995, . 4030 [21] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4031 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4032 DOI 10.17487/RFC1869, November 1995, 4033 . 4035 [22] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4036 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4037 . 4039 [23] De Winter, J., "SMTP Service Extension for Remote Message 4040 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4041 1996, . 4043 [24] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4044 Extensions (MIME) Part One: Format of Internet Message 4045 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4046 . 4048 [25] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4049 Part Three: Message Header Extensions for Non-ASCII Text", 4050 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4051 . 4053 [26] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4054 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4055 DOI 10.17487/RFC2156, January 1998, 4056 . 4058 [27] Elz, R. and R. Bush, "Clarifications to the DNS 4059 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4060 . 4062 [28] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4063 Word Extensions: Character Sets, Languages, and 4064 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4065 1997, . 4067 [29] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4068 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4069 . 4071 [30] Freed, N., "SMTP Service Extension for Command 4072 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4073 September 2000, . 4075 [31] Freed, N., "Behavior of and Requirements for Internet 4076 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4077 . 4079 [32] Vaudreuil, G., "SMTP Service Extensions for Transmission 4080 of Large and Binary MIME Messages", RFC 3030, 4081 DOI 10.17487/RFC3030, December 2000, 4082 . 4084 [33] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4085 Extension for Delivery Status Notifications (DSNs)", 4086 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4087 . 4089 [34] Vaudreuil, G., "Enhanced Mail System Status Codes", 4090 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4091 . 4093 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4094 for Delivery Status Notifications", RFC 3464, 4095 DOI 10.17487/RFC3464, January 2003, 4096 . 4098 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4099 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4100 . 4102 [37] Hansen, T., Ed. and G. Vaudreuil, Ed., "Message 4103 Disposition Notification", RFC 3798, DOI 10.17487/RFC3798, 4104 May 2004, . 4106 [38] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 4107 Extensions (S/MIME) Version 3.1 Message Specification", 4108 RFC 3851, DOI 10.17487/RFC3851, July 2004, 4109 . 4111 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4112 in Mixed IPv4/v6 Environments", RFC 3974, 4113 DOI 10.17487/RFC3974, January 2005, 4114 . 4116 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4117 Resource Identifier (URI): Generic Syntax", STD 66, 4118 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4119 . 4121 [41] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 4122 for Authorizing Use of Domains in E-Mail, Version 1", 4123 RFC 4408, DOI 10.17487/RFC4408, April 2006, 4124 . 4126 [42] Gellens, R. and J. Klensin, "Message Submission for Mail", 4127 RFC 4409, DOI 10.17487/RFC4409, April 2006, 4128 . 4130 [43] Fenton, J., "Analysis of Threats Motivating DomainKeys 4131 Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, 4132 September 2006, . 4134 [44] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 4135 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 4136 Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007, 4137 . 4139 [45] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4140 Thayer, "OpenPGP Message Format", RFC 4880, 4141 DOI 10.17487/RFC4880, November 2007, 4142 . 4144 [46] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4145 Mail System Status Codes", BCP 138, RFC 5248, 4146 DOI 10.17487/RFC5248, June 2008, 4147 . 4149 [47] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4150 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4151 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4152 . 4154 [48] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4155 DOI 10.17487/RFC7504, June 2015, 4156 . 4158 [49] Internet Assigned Number Authority (IANA), "IANA Mail 4159 Parameters", 2007, 4160 . 4162 [50] Internet Assigned Number Authority (IANA), "Address 4163 Literal Tags", 2007, 4164 . 4166 [51] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4167 Record for Domains that Accept No Mail", September 2014, 4168 . 4171 [52] RFC Editor, "RFC Errata - RFC 5321", 2019, 4172 . 4174 Captured 2019-11-19 4176 Appendix A. TCP Transport Service 4178 The TCP connection supports the transmission of 8-bit bytes. The 4179 SMTP data is 7-bit ASCII characters. Each character is transmitted 4180 as an 8-bit byte with the high-order bit cleared to zero. Service 4181 extensions may modify this rule to permit transmission of full 8-bit 4182 data bytes as part of the message body, or, if specifically designed 4183 to do so, in SMTP commands or responses. 4185 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4187 Some systems use an RFC 822 header section (only) in a mail 4188 submission protocol, or otherwise generate SMTP commands from RFC 822 4189 header fields when such a message is handed to an MTA from a UA. 4190 While the MTA-UA protocol is a private matter, not covered by any 4191 Internet Standard, there are problems with this approach. For 4192 example, there have been repeated problems with proper handling of 4193 "bcc" copies and redistribution lists when information that 4194 conceptually belongs to the mail envelope is not separated early in 4195 processing from header field information (and kept separate). 4197 It is recommended that the UA provide its initial ("submission 4198 client") MTA with an envelope separate from the message itself. 4199 However, if the envelope is not supplied, SMTP commands SHOULD be 4200 generated as follows: 4202 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4203 be copied to a RCPT command (generating multiple message copies 4204 if that is required for queuing or delivery). This includes any 4205 addresses listed in a RFC 822 "group". Any BCC header fields 4206 SHOULD then be removed from the header section. Once this 4207 process is completed, the remaining header fields SHOULD be 4208 checked to verify that at least one TO, CC, or BCC header field 4209 remains. If none do, then a BCC header field with no additional 4210 information SHOULD be inserted as specified in [11]. 4212 2. The return address in the MAIL command SHOULD, if possible, be 4213 derived from the system's identity for the submitting (local) 4214 user, and the "From:" header field otherwise. If there is a 4215 system identity available, it SHOULD also be copied to the Sender 4216 header field if it is different from the address in the From 4217 header field. (Any Sender header field that was already there 4218 SHOULD be removed.) Systems may provide a way for submitters to 4219 override the envelope return address, but may want to restrict 4220 its use to privileged users. This will not prevent mail forgery, 4221 but may lessen its incidence; see Section 7.1. 4223 When an MTA is being used in this way, it bears responsibility for 4224 ensuring that the message being transmitted is valid. The mechanisms 4225 for checking that validity, and for handling (or returning) messages 4226 that are not valid at the time of arrival, are part of the MUA-MTA 4227 interface and not covered by this specification. 4229 A submission protocol based on Standard RFC 822 information alone 4230 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4231 system into an SMTP environment. Additional information to construct 4232 an envelope must come from some source in the other environment, 4233 whether supplemental header fields or the foreign system's envelope. 4235 Attempts to gateway messages using only their header "To" and "Cc" 4236 fields have repeatedly caused mail loops and other behavior adverse 4237 to the proper functioning of the Internet mail environment. These 4238 problems have been especially common when the message originates from 4239 an Internet mailing list and is distributed into the foreign 4240 environment using envelope information. When these messages are then 4241 processed by a header-section-only remailer, loops back to the 4242 Internet environment (and the mailing list) are almost inevitable. 4244 Appendix C. Source Routes 4246 Historically, the was a reverse source routing list of 4247 hosts and a source mailbox. The first host in the was 4248 historically the host sending the MAIL command; today, source routes 4249 SHOULD NOT appear in the reverse-path. Similarly, the 4250 may be a source routing lists of hosts and a destination mailbox. 4251 However, in general, the SHOULD contain only a mailbox 4252 and domain name, relying on the domain name system to supply routing 4253 information if required. The use of source routes is deprecated (see 4254 Appendix F.2); while servers MUST be prepared to receive and handle 4255 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT 4256 transmit them and this section is included in the current 4257 specification only to provide context. It has been modified somewhat 4258 from the material in RFC 821 to prevent server actions that might 4259 confuse clients or subsequent servers that do not expect a full 4260 source route implementation. 4262 Historically, for relay purposes, the forward-path may have been a 4263 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4264 THREE MUST be fully-qualified domain names. This form was used to 4265 emphasize the distinction between an address and a route. The 4266 mailbox (here, JOE@THREE) is an absolute address, and the route is 4267 information about how to get there. The two concepts should not be 4268 confused.[[CREF28: [5321bis]JcK 20090123: Tightened this and the next 4269 paragraph to be clear that this doesn't authorize source route use.]] 4270 If source routes are used contrary to requirements and 4271 recommendations elsewhere in this specfiication, RFC 821 and the text 4272 below should be consulted for the mechanisms for constructing and 4273 updating the forward-path. A server that is reached by means of a 4274 source route (e.g., its domain name appears first in the list in the 4275 forward-path) MUST remove its domain name from any forward-paths in 4276 which that domain name appears before forwarding the message and MAY 4277 remove all other source routing information. The reverse-path SHOULD 4278 NOT be updated by servers conforming to this specification. 4280 Notice that the forward-path and reverse-path appear in the SMTP 4281 commands and replies, but not necessarily in the message. That is, 4282 there is no need for these paths and especially this syntax to appear 4283 in the "To:" , "From:", "CC:", etc. fields of the message header 4284 section. Conversely, SMTP servers MUST NOT derive final message 4285 routing information from message header fields. 4287 When the list of hosts is present despite the recommendations and 4288 requirements [[CREF29: [5321bis]JcK 20090123 "and requrements" 4289 added]] above, it is a "reverse" source route and indicates that the 4290 mail was relayed through each host on the list (the first host in the 4291 list was the most recent relay). This list is used as a source route 4292 to return non-delivery notices to the sender. If, contrary to the 4293 recommendations here, a relay host adds itself to the beginning of 4294 the list, it MUST use its name as known in the transport environment 4295 to which it is relaying the mail rather than that of the transport 4296 environment from which the mail came (if they are different). Note 4297 that a situation could easily arise in which some relay hosts add 4298 their names to the reverse source route and others do not, generating 4299 discontinuities in the routing list. This is another reason why 4300 servers needing to return a message SHOULD ignore the source route 4301 entirely and simply use the domain as specified in the Mailbox. 4303 Appendix D. Scenarios 4305 This section presents complete scenarios of several types of SMTP 4306 sessions. In the examples, "C:" indicates what is said by the SMTP 4307 client, and "S:" indicates what is said by the SMTP server. 4309 D.1. A Typical SMTP Transaction Scenario 4311 This SMTP example shows mail sent by Smith at host bar.com, and to 4312 Jones, Green, and Brown at host foo.com. Here we assume that host 4313 bar.com contacts host foo.com directly. The mail is accepted for 4314 Jones and Brown. Green does not have a mailbox at host foo.com. 4316 S: 220 foo.com Simple Mail Transfer Service Ready 4317 C: EHLO bar.com 4318 S: 250-foo.com greets bar.com 4319 S: 250-8BITMIME 4320 S: 250-SIZE 4321 S: 250-DSN 4322 S: 250 HELP 4323 C: MAIL FROM: 4324 S: 250 OK 4325 C: RCPT TO: 4326 S: 250 OK 4327 C: RCPT TO: 4328 S: 550 No such user here 4329 C: RCPT TO: 4330 S: 250 OK 4331 C: DATA 4332 S: 354 Start mail input; end with . 4333 C: Blah blah blah... 4334 C: ...etc. etc. etc. 4335 C: . 4336 S: 250 OK 4337 C: QUIT 4338 S: 221 foo.com Service closing transmission channel 4340 D.2. Aborted SMTP Transaction Scenario 4342 S: 220 foo.com Simple Mail Transfer Service Ready 4343 C: EHLO bar.com 4344 S: 250-foo.com greets bar.com 4345 S: 250-8BITMIME 4346 S: 250-SIZE 4347 S: 250-DSN 4348 S: 250 HELP 4349 C: MAIL FROM: 4350 S: 250 OK 4351 C: RCPT TO: 4352 S: 250 OK 4353 C: RCPT TO: 4354 S: 550 No such user here 4355 C: RSET 4356 S: 250 OK 4357 C: QUIT 4358 S: 221 foo.com Service closing transmission channel 4360 D.3. Relayed Mail Scenario 4362 Step 1 -- Source Host to Relay Host 4364 The source host performs a DNS lookup on XYZ.COM (the destination 4365 address) and finds DNS MX records specifying xyz.com as the best 4366 preference and foo.com as a lower preference. It attempts to open a 4367 connection to xyz.com and fails. It then opens a connection to 4368 foo.com, with the following dialogue: 4370 S: 220 foo.com Simple Mail Transfer Service Ready 4371 C: EHLO bar.com 4372 S: 250-foo.com greets bar.com 4373 S: 250-8BITMIME 4374 S: 250-SIZE 4375 S: 250-DSN 4376 S: 250 HELP 4377 C: MAIL FROM: 4378 S: 250 OK 4379 C: RCPT TO: 4380 S: 250 OK 4381 C: DATA 4382 S: 354 Start mail input; end with . 4383 C: Date: Thu, 21 May 1998 05:33:29 -0700 4384 C: From: John Q. Public 4385 C: Subject: The Next Meeting of the Board 4386 C: To: Jones@xyz.com 4387 C: 4388 C: Bill: 4389 C: The next meeting of the board of directors will be 4390 C: on Tuesday. 4391 C: John. 4392 C: . 4393 S: 250 OK 4394 C: QUIT 4395 S: 221 foo.com Service closing transmission channel 4397 Step 2 -- Relay Host to Destination Host 4399 foo.com, having received the message, now does a DNS lookup on 4400 xyz.com. It finds the same set of MX records, but cannot use the one 4401 that points to itself (or to any other host as a worse preference). 4402 It tries to open a connection to xyz.com itself and succeeds. Then 4403 we have: 4405 S: 220 xyz.com Simple Mail Transfer Service Ready 4406 C: EHLO foo.com 4407 S: 250 xyz.com is on the air 4408 C: MAIL FROM: 4409 S: 250 OK 4410 C: RCPT TO: 4411 S: 250 OK 4412 C: DATA 4413 S: 354 Start mail input; end with . 4414 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4415 C: 05:33:29 -0700 4416 C: Date: Thu, 21 May 1998 05:33:29 -0700 4417 C: From: John Q. Public 4418 C: Subject: The Next Meeting of the Board 4419 C: To: Jones@xyz.com 4420 C: 4421 C: Bill: 4422 C: The next meeting of the board of directors will be 4423 C: on Tuesday. 4424 C: John. 4425 C: . 4426 S: 250 OK 4427 C: QUIT 4428 S: 221 xyz.com Service closing transmission channel 4430 D.4. Verifying and Sending Scenario 4432 S: 220 foo.com Simple Mail Transfer Service Ready 4433 C: EHLO bar.com 4434 S: 250-foo.com greets bar.com 4435 S: 250-8BITMIME 4436 S: 250-SIZE 4437 S: 250-DSN 4438 S: 250-VRFY 4439 S: 250 HELP 4440 C: VRFY Crispin 4441 S: 250 Mark Crispin 4442 C: MAIL FROM: 4443 S: 250 OK 4444 C: RCPT TO: 4445 S: 250 OK 4446 C: DATA 4447 S: 354 Start mail input; end with . 4448 C: Blah blah blah... 4449 C: ...etc. etc. etc. 4450 C: . 4451 S: 250 OK 4452 C: QUIT 4453 S: 221 foo.com Service closing transmission channel 4455 Appendix E. Other Gateway Issues 4457 In general, gateways between the Internet and other mail systems 4458 SHOULD attempt to preserve any layering semantics across the 4459 boundaries between the two mail systems involved. Gateway- 4460 translation approaches that attempt to take shortcuts by mapping 4461 (such as mapping envelope information from one system to the message 4462 header section or body of another) have generally proven to be 4463 inadequate in important ways. Systems translating between 4464 environments that do not support both envelopes and a header section 4465 and Internet mail must be written with the understanding that some 4466 information loss is almost inevitable. 4468 Appendix F. Deprecated Features of RFC 821 4470 A few features of RFC 821 have proven to be problematic and SHOULD 4471 NOT be used in Internet mail. Some of these features were deprecated 4472 in RFC 2821 in 2001; source routing and two-digit years in dates were 4473 deprecated by RFC 1123 in 1989. Of the domain literal forms, RFC 4474 1123 required support only for the dotted decimal form. With the 4475 possible exception of old, hardware-embedded, applications, there is 4476 no longer any excuse for these features to appear on the contemporary 4477 Internet. [[CREF30: [5321bis] (2821ter) 2821bis Last Call Comment]] 4479 F.1. TURN 4481 This command, described in RFC 821, raises important security issues 4482 since, in the absence of strong authentication of the host requesting 4483 that the client and server switch roles, it can easily be used to 4484 divert mail from its correct destination. Its use is deprecated; 4485 SMTP systems SHOULD NOT use it unless the server can authenticate the 4486 client. 4488 F.2. Source Routing 4490 RFC 821 utilized the concept of explicit source routing to get mail 4491 from one host to another via a series of relays. The requirement to 4492 utilize source routes in regular mail traffic was eliminated by the 4493 introduction of the domain name system "MX" record and the last 4494 significant justification for them was eliminated by the 4495 introduction, in RFC 1123, of a clear requirement that addresses 4496 following an "@" must all be fully-qualified domain names. 4497 Consequently, the only remaining justifications for the use of source 4498 routes are support for very old SMTP clients or MUAs and in mail 4499 system debugging. They can, however, still be useful in the latter 4500 circumstance and for routing mail around serious, but temporary, 4501 problems such as problems with the relevant DNS records. 4503 SMTP servers MUST continue to accept source route syntax as specified 4504 in the main body of this document and in RFC 1123. They MAY, if 4505 necessary, ignore the routes and utilize only the target domain in 4506 the address. If they do utilize the source route, the message MUST 4507 be sent to the first domain shown in the address. In particular, a 4508 server MUST NOT guess at shortcuts within the source route. 4510 Clients SHOULD NOT utilize explicit source routing except under 4511 unusual circumstances, such as debugging or potentially relaying 4512 around firewall or mail system configuration errors. 4514 F.3. HELO 4516 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4517 than HELO when the server will accept the former. Servers MUST 4518 continue to accept and process HELO in order to support older 4519 clients. 4521 F.4. #-literals 4523 RFC 821 provided for specifying an Internet address as a decimal 4524 integer host number prefixed by a pound sign, "#". In practice, that 4525 form has been obsolete since the introduction of TCP/IP. It is 4526 deprecated and MUST NOT be used. 4528 F.5. Dates and Years 4530 When dates are inserted into messages by SMTP clients or servers 4531 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4532 digit years are deprecated; three-digit years were never permitted in 4533 the Internet mail system. 4535 F.6. Sending versus Mailing 4537 In addition to specifying a mechanism for delivering messages to 4538 user's mailboxes, RFC 821 provided additional, optional, commands to 4539 deliver messages directly to the user's terminal screen. These 4540 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4541 workstation technology and the introduction of other protocols may 4542 have rendered them obsolete even where they are implemented. 4543 [[5321bis Editor's Note: does this need a stronger reference to 821, 4544 2821, and/or 5321?]] 4546 Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers 4547 MAY implement them. If they are implemented by servers, the 4548 implementation model specified in RFC 821 MUST be used and the 4549 command names MUST be published in the response to the EHLO command. 4551 Appendix G. Other Outstanding Issues 4553 [[RFC Editor: Please remove this section before publication.]] 4555 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4556 that led to a broad discussion of ways in which existing practice had 4557 diverged from the specifications and recommendations of RFC 5321 in 4558 the more than eleven years since it was published (some of those 4559 issues probably affect the boundary between RFC 5321 and 5322 and 4560 hence the latter as well). In most cases, those divergences call for 4561 revision of the Technical Specification to match the practice, 4562 clarification of the specification text in other ways, or a more 4563 comprehensive explanation of why the practices recommended by the 4564 specification should really be followed. 4566 Those discussions raised two other issues, which were that 4568 o The publication of the Submission Server specification of RFC 6409 4569 in November 2011 may not have been fully reflected in RFC 5321 4570 (despite the even earlier publication of RFC 4409) and 4572 o There may be inconsistencies between the July 2009 Internet Mail 4573 Architecture description of RFC 5598 and the model described in 4574 RFC 5321. The issue called out in Appendix G.3 below may be an 4575 example of one of those inconsistencies. 4577 Those discrepancies should be identified and discussed and decisions 4578 made to fix them (and where) or to ignore them and let them continue. 4580 There has also been discussion on the mailing list, perhaps amounting 4581 to very rough consensus, that any revision of RFC 5321 and/or 5322 4582 should be accompanied by a separate Applicability Statement document 4583 that would make recommendations about applicability or best practices 4584 in particular areas rather than trying to get everything into the two 4585 technical specifications. This appendix does not attempt to identify 4586 which issues should get which treatment. 4588 Until and unless there is a WG with appropriate leadership and 4589 tracking mechanisms, this appendix will act as a temporary record of 4590 issues that should be discussed and decided upon before a revised 4591 SMTP specification (or a related Applicability Statement) is 4592 published, issues that have not been reflected in errata (see 4593 Appendix H.1 below for those covered by errata). 4595 G.1. IP Address Literals 4597 The specification is unclear about whether IP address literals, 4598 particularly IP address literals used as arguments to the EHLO 4599 command, are required to be accepted or whether they are allowed to 4600 be rejected as part of the general "operational necessity" exception. 4601 Some have suggested that rejection of them is so common as an anti- 4602 spam measure that the use of such literals should be deprecated 4603 entirely in the specification, others that the are still useful and 4604 used and/or that, whatever is said about IP address literals within 4605 an SMTP session (e.g., in MAIL or RCPT commands), they should 4606 continue to be allowed (and required) in EHLO. 4608 G.2. Repeated Use of EHLO 4610 While the specification says that an SMTP client's sending EHLO again 4611 after it has been issued (starting an SMTP session and treats it as 4612 if RSET had been sent (closing the session) followed by EHLO, there 4613 are apparently applications, at least some of them involving setting 4614 up of secure connections, in which the second EHLO is required and 4615 does not imply RSET. Does the specification need to be adjusted to 4616 reflect or call out those cases? 4618 G.3. Meaning of "MTA" and Related Terminology 4620 A terminology issue has come up about what the term "MTA" actually 4621 refers to, a question that became at least slightly more complicated 4622 when we formalized RFC 6409 Submission Servers. Does the document 4623 need to be adjusted to be more clear about this topic? Note that the 4624 answer may interact with the question asked in Section 2 above. 4625 Possibly along the same lines, RFC 2821 changed the RFC 821 4626 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4627 and "SMTP server" respectively. As things have evolved, it is 4628 possible that newer terminology is a source of confusion and that the 4629 terminology should be changed back, something that also needs 4630 discussion. 4632 G.4. Originator, or Originating System, Authentication 4634 Should RFC 5321bis address authentication and related issues or 4635 should Section 3.9 or other text be reshaped (in addition to or 4636 instead of the comment on that section) to lay a better foundation 4637 for such work, either in the context of mailing lists or more 4638 generally? 4640 G.5. Remove or deprecate the work-around from code 552 to 452 4642 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4643 and/or be inconsistent with current practice. Should it be removed 4644 and/or explicitly deprecated? 4646 G.6. Clarify where the protocol stands with respect to submission and 4647 TLS issues 4649 1. submission on port 587 4651 2. submission on port 465 4653 3. TLS relay on a port different from 25 (whenever) 4655 4. Recommendations about general use of transport layer (hop by hop) 4656 security, particularly encryption including consideration of RFC 4657 8314. 4659 G.7. Probably-substantive Discussion Topics Identified in Other Ways 4661 The following issues were identified as a group in the opening Note 4662 but called out specifically only in embedded CREF comments in earlier 4663 (-00 and -01) versions of this draft. 4665 G.7.1. Issues with 521, 554, and 556 codes 4667 See new Section 4.2.4.2. More text may be needed, there or 4668 elsewhere, about choices of codes in response to initial opening and 4669 to EHLO, especially to deal with selective policy rejections. 4671 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 4673 CREF comment in Section 2 and also CREF comment in Section 2.3.10 4675 G.7.3. Resolvable FQDNs and private domain names 4677 Multiple CREF comments in Section 2.3.5 4679 G.7.4. Possible clarification about mail transactions and transaction 4680 state 4682 CREF comment in Section 3.3 and also reference in Section 4.1.4 4684 G.7.5. Issues with mailing lists, aliases, and forwarding 4686 CREF comment in Section 3.9. May also want to note forwarding as an 4687 email address portability issue. 4689 G.7.6. Requirements for domain name and/or IP address in EHLO 4691 CREF comment in Section 4.1.4 4693 G.7.7. Does the 'first digit only' and/or non-listed reply code text 4694 need clarification? 4696 CREF comments in Section 4.2 and Section 4.3.1 4698 G.7.8. Size limits 4700 CREF comment in Section 4.5.3 4702 G.7.9. Discussion of 'blind' copies and RCPT 4704 CREF comment in Section 7.2. May alto need to discussion whether 4705 that terminology is politically incorrect and suggest a replacement. 4707 G.7.10. Further clarifications needed to source routes? 4709 CREF comment in Appendix C 4711 G.7.11. Should 1yz Be Revisited? 4713 RFC 5421 depreciated the "positive preliminary reply" response code 4714 category with first digit "1", so that the first digit of valid SMTP 4715 response codes must be 2, 3, 4, or 5. It has been suggested (see 4716 mail from Hector Santos with Subject "SMTP Reply code 1yz Positive 4717 Preliminary reply", March 5, 2020 12:56 -0500, on the SMTP list) that 4718 these codes should be reinstated to deal with some situations that 4719 became more plausible after 5321 was published. Do we need to take 4720 this back up? 4722 G.7.12. Review Timeout Specifications 4724 RFC 5321 (and its predecessors going back to 821) specify minimum 4725 periods for client and server to wait before timing out. Are those 4726 intervals still appropriate in a world of faster processors and 4727 faster networks? Should they be updated and revised? Or should more 4728 qualifying language be added? 4730 G.8. Enhanced Reply Codes and DSNs 4732 Enhanced Mail System Status Codes [34] were added to SMTP before RFC 4733 5321 was published and are now, together with a corresponding 4734 registry [46], widely deployed and in extensive use in the network. 4735 Similar, the structure and extensions options for Delivery Status 4736 Notifications [35] is implemented, deployed, and in wide use. Is it 4737 time to fold all or part of those mature specifications into the SMTP 4738 spec or at least to mention and normatively reference them? And, as 4739 an aside do those specs need work or, if they are kept separate, is 4740 it time to move them to Internet Standard? 4742 G.9. Revisiting Quoted Strings 4744 Recent discussions both in and out of the IETF have highlighted 4745 instances of non-compliance with the specification of a Local-part 4746 consisting of a Quoted-string, whether any content of QcontentSMTP 4747 that actually requires special treatment consists of qtextSMTP, 4748 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4749 above) ends with a few paragraphs of warnings (essentially a partial 4750 applicability statement), the first of which cautions against 4751 cleverness with either Quoted-string or case sensitivity as a threat 4752 to interoperability. 4754 The Quoted-string portion of that discussion has apparently been 4755 widely not read or ignored. Do we need to do something else? If we 4756 do an Applicability Statement, would it be useful to either reference 4757 the discussion in this document from there or to move the discussion 4758 there and reference it (normatively?) from here? 4760 There has been a separate discussion of empty quoted strings in 4761 addresses, i.e., whether the production should be 4762 required to included at least one non-whitespace character. It is 4763 separate from this issue but would be further impacted or distorted 4764 from the considerations identified in this Section. 4766 G.10. Internationalization 4768 RFC 5321 came long before work on internationalization of email 4769 addresses and headers (other than by use of encoded words in MINE) 4770 and specifically before the work of the EAI WG leading to the 4771 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4772 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4773 define mailboxes ...") is an extremely strong prohibition against the 4774 use of non-ASCII characters. Would it be appropriate to add 4775 something like "in the absence of relevant extensions" there? Also, 4776 given [mis]behavior seen in the wild, does that paragraph (or an A/S) 4777 need an explicit caution about SMTP servers or clients assuming they 4778 can apply the popular web convention of using %NN sequences as a way 4779 to encode non-ASCII characters ( in RFC 3986) and 4780 assuming some later system will interpret it as they expect? Would 4781 it be appropriate to add an Internationalization Considerations 4782 section to the body of this document if only for the purpose of 4783 pointing people elsewhere? 4785 G.11. SMTP Clients, Servers, Senders, and Receivers 4787 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4788 2821 (and hence in 5321), we switched that to "client" and "server" 4789 (See the discussion in Section 1.2). In part because a relay is a 4790 server and then a client (in some recent practice, even interleaving 4791 the two functions by opening the connection to the next host in line 4792 and sending commands before the incoming transaction is complete), 4793 RFC 5321 continues to use the original terminology in some places. 4794 Should we revisit that usage, possibly even returning to consistent 4795 use of the original terminology? 4797 Appendix H. Change log for RFC 5321bis 4799 [[RFC Editor: Please remove this section before publication.]] 4801 H.1. RFC 5321 Errata Summary 4803 This document addresses the following errata filed against RFC 5321 4804 since its publication in October 2008 [52] [[CREF31: [[Note in Draft: 4805 Items with comments below have not yet been resolved.]]]] 4807 1683 ABNF error. Section 4.4 4809 4198 Description error. Section 4.2 4811 2578 Syntax description error. Section 4.1.2 4813 1543 Wrong code in description Section 3.8 4815 4315 ABNF - IPv6 Section 4.1.3. [[CREF32: [5321bis]The IPv6 syntax 4816 has been adjusted since 5321 was published. See the rewritten 4817 form and the comment in the section cited in the previous 4818 sentence. The editor awaits instructions. See https://www.rfc- 4819 editor.org/errata/eid4315]] 4821 5414 ABNF for Quoted-string Section 4.1.2 4823 1851 Location of text on unexpected close Section 4.1.1.5. 4824 [[CREF33: [5321bis]Matter of taste, editor seeks advice.]] 4826 3447 Use of normative language (e.g., more "MUST"s), possible 4827 confusion in some sections Section 4.4. [[CREF34: [5321bis]As 4828 Barry notes in his verifier comments on the erratum (see 4829 https://www.rfc-editor.org/errata/eid3447), the comments and 4830 suggestions here raise a number of interesting (and difficult) 4831 issues. One of the issues is that the core of RFCs 5321 (and 4832 2821) is text carried over from Jon Postel's RFC 821, a document 4833 that was not only written in a different style than the IETF uses 4834 today but that was written at a time when no one had dreamt of RFC 4835 2119 or even the IETF itself. It appears to me that trying to 4836 patch that style might easily result in a document that is harder 4837 to read as well as being error prone. If we want to get the 4838 document entirely into contemporary style, we really should bite 4839 the bullet and do a complete rewrite. To respond to a different 4840 point in Barry's discussion, I think an explicit statement that 4841 5321/5322 and their predecessors differ in places and why would be 4842 helpful. Text, and suggestions about where to put it, are 4843 solicited. A list of differences might be a good idea too, but 4844 getting it right might be more work than there is available energy 4845 to do correctly. ]] 4847 5711 Missing leading spaces in example Appendix D.3. [[CREF35: 4848 [5321bis]Well, this is interesting because the XML is correct and 4849 the spaces are there, embedded in artwork. So either the XML2RFC 4850 processor at the time took those leading spaces out or the RFC 4851 Editor improved on the document and the change was not caught in 4852 AUTH48, perhaps because rfcdiff ignores white space. We just need 4853 to watch for future iterations. ]] 4855 [[CREF36: [5321bis]Note that rejected errata have _not_ been reviewed 4856 to see if they contain anything useful that should be discussed again 4857 with the possibility of rethinking and changing text. Volunteers 4858 sought.]] 4860 H.2. Changes from RFC 5321 (published October 2008) to the initial 4861 (-00) version of this draft 4863 o Acknowledgments section (Section 9) trimmed back for new document. 4865 o Introductory paragraph to Appendix F extended to make it clear 4866 that these features were deprecated a long time ago and really 4867 should not be in use any more. 4869 o Adjusted some language to clarify that source routes really, 4870 really, should not be used or depended upon. 4872 o IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 4873 a note added. 4875 o Production index added as a first step in tying all productions to 4876 their sources. As part of the effort to make the document more 4877 easily navigable, table of contents entries have been created for 4878 the individual command descriptions. 4880 o Clarified the relationship between the SMTP "letters, digits, and 4881 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 4883 o Revised the reply code sections to add new 521 and 556 codes, 4884 clarify relationships, and be explicit about the requirement for 4885 clients to rely on first digits rather than the sequences in 4886 Section 4.3.2. 4888 o In conjunction with the above, explicitly obsolete RFCs 1846 and 4889 7504. 4891 o Incorporated a correction reflecting Errata ID 2578. 4893 o Some small editorial changes made to eliminate redundant 4894 statements that were very close together. Other, equally small, 4895 editorial changes have been made to improve grammar or clarity. 4897 o A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 4898 Draft" have been added for the group to resolve. Other questions, 4899 especially those in the errata summary, are simply included in 4900 narrative comments in CREFs. 4902 o Checked and rationalized "response" (to a command) and "reply 4903 code" terminology. One can talk about a "999 response" but only a 4904 "999 reply code". There is no such thing as a "response code". 4906 o Added note about length limit on mailbox names ("email 4907 addresses"). 4909 o Added an "errata summary" subsection to this change log/ 4910 comparison to 5321 in this Appendix. The entire Appendix will, of 4911 course, disappear at the time of RFC publication unless someone 4912 wants to make a strong case for retaining it. 4914 o Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 4915 readers below the Abstract. 4917 o Temporarily added a "Note on Reading This Working Draft" after the 4918 Abstract. 4920 H.3. Changes Among Versions of Rfc5321bis 4922 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 4923 -01 4925 Substantively, these two versions differ only by suppression of the 4926 CREF and other discussion associated with the evolution from RFC 2821 4927 to RFC 5321. That change includes an update to the document's Note 4928 to Readers, the date, the file name, and the addition of this change 4929 log subsection. 4931 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 4933 o Minor clarifications to improve text, e.g., addition of NOOP to 4934 the list of non-mail transaction examples in Section 4.1.4. 4936 o Added topics exposed in the ietf-smtp list and the IETF list 4937 "dogfood" discussion during December 2019 and an index listing of 4938 substantive issues identified only in CREFs in the prior draft as 4939 a new Appendix G.. 4941 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 4943 o Added more text to Appendix G.7.1 to specifically call out the 4944 session-opening policy issues surrounding these codes. 4946 o Added discussion of "1yz" reinstatement in Appendix G.7.11. 4948 o Added discussion of timeouts in Appendix G.7.12. 4950 o Added subsection on Enhanced Status Codes and DSNs to the 4951 outstanding issues list Appendix G.8. 4953 o Replaced reference to RFC 1652 (8BITMIME) with the Internet 4954 Standard version, RFC 6152. 4956 o With help from cketti, clarified the ABNF productions whose 4957 terminals appear in other documents. 4959 o Added discussions of Quoted-string, Internationalization, and 4960 client-server versus sender-receiver terminology to Appendix G. 4962 o Added note to the Abstract. 4964 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) to draft- 4965 ietf-emailcore-rfc5321bis-00 4967 o Added a paragraph about non-null quoted strings to Appendix G.9. 4969 o Added an explicit pointer to email insecurity and TLS to 4970 Appendix G.6. Inspired by Ben Kaduk's comment on the WG Charter, 4971 2020-09-09. 4973 o Converted document from individual to emailcore WG effort. 4975 Index 4977 A 4978 Argument Syntax 4979 A-d-l 42 4980 Additional-Registered-Clauses 63 4981 address-literal 43 4982 Addtl-Link 63 4983 Addtl-Protocol 63 4984 ALPHA 42 4985 Argument 42 4986 At-domain 42 4987 atext 42 4988 Atom 43 4989 By-domain 62 4990 CFWS 42 4991 CRLF 42 4992 dcontent 45 4993 DIGIT 42 4994 Domain 43 4995 Dot-string 43 4996 esmtp-keyword 42 4997 esmtp-param 42 4998 esmtp-value 42 4999 Extended-Domain 62 5000 For 63 5001 Forward-Path 42 5002 From-domain 62 5003 FWS 42 5004 General-address-literal 45 5005 Greeting 48 5006 h16 45 5007 HEXDIG 42 5008 ID 63 5009 IPv4-address-literal 45 5010 IPv6-addr 45 5011 IPv6-address-literal 45 5012 Keyword 42 5013 Ldh-str 43 5014 Let-dig 43 5015 Link 63 5016 Local-part 43 5017 ls32 45 5018 Mail-parameters 42 5019 Mailbox 43 5020 Opt-info 62 5021 Path 42 5022 Protocol 63 5023 QcontentSMTP 43 5024 qtextSMTP 43 5025 quoted-pairSMTP 43 5026 Quoted-string 43 5027 Rcpt-parameters 42 5028 Reply-code 48 5029 Reply-line 48 5030 Return-path-line 62 5031 Reverse-Path 42 5032 Snum 45 5033 SP 42 5034 Stamp 62 5035 Standardized-tag 45 5036 String 43 5037 sub-domain 43 5038 TCP-info 62 5039 textstring 48 5040 Time-stamp-line 62 5041 Via 62 5042 With 62 5044 C 5045 Command Syntax 5046 data 39 5047 expn 40 5048 help 40 5049 mail 36 5050 noop 41 5051 quit 41 5052 rcpt 38 5053 rset 39 5054 vrfy 40 5056 Author's Address 5058 John C. Klensin 5059 1770 Massachusetts Ave, Suite 322 5060 Cambridge, MA 02140 5061 USA 5063 EMail: john-ietf@jck.com