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'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 Network Working Group J. Klensin 3 Internet-Draft July 2, 2020 4 Obsoletes: 5321, 1846, 7504 (if 5 approved) 6 Updates: 1123 (if approved) 7 Intended status: Standards Track 8 Expires: January 3, 2021 10 Simple Mail Transfer Protocol 11 draft-klensin-rfc5321bis-03 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 January 3, 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 . 99 212 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 . . . . . . . . . . . . . . 100 222 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 101 223 G.10. Internationalization . . . . . . . . . . . . . . . . . . 101 224 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 101 225 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 . . . . . . . . . . 104 230 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 231 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 104 232 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) 233 to -02 . . . . . . . . . . . . . . . . . . . . . . . 104 234 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 235 to -03 . . . . . . . . . . . . . . . . . . . . . . . 105 236 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 237 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 107 239 1. Introduction 241 1.1. Transport of Electronic Mail 243 The objective of the Simple Mail Transfer Protocol (SMTP) is to 244 transfer mail reliably and efficiently. 246 SMTP is independent of the particular transmission subsystem and 247 requires only a reliable ordered data stream channel. While this 248 document specifically discusses transport over TCP, other transports 249 are possible. Appendices to RFC 821 [3] describe some of them. 251 An important feature of SMTP is its capability to transport mail 252 across multiple networks, usually referred to as "SMTP mail relaying" 253 (see Section 3.6). A network consists of the mutually-TCP-accessible 254 hosts on the public Internet, the mutually-TCP-accessible hosts on a 255 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 256 environment utilizing a non-TCP transport-level protocol. Using 257 SMTP, a process can transfer mail to another process on the same 258 network or to some other network via a relay or gateway process 259 accessible to both networks. 261 In this way, a mail message may pass through a number of intermediate 262 relay or gateway hosts on its path from sender to ultimate recipient. 263 The Mail eXchanger mechanisms of the domain name system (RFC 1035 264 [4], RFC 974 [15], and Section 5 of this document) are used to 265 identify the appropriate next-hop destination for a message being 266 transported. 268 1.2. History and Context for This Document 270 This document is a specification of the basic protocol for the 271 Internet electronic mail transport. It consolidates, updates and 272 clarifies, but does not add new or change existing functionality of 273 the following: 275 o the original SMTP (Simple Mail Transfer Protocol) specification of 276 RFC 821 [3], 278 o domain name system requirements and implications for mail 279 transport from RFC 1035 [4] and RFC 974 [15], 281 o the clarifications and applicability statements in RFC 1123 [5], 283 o the new error codes added by RFC 1846 [19] and later by RFC 7504 284 [48], obsoleting both of those documents, and 286 o material drawn from the SMTP Extension mechanisms in RFC 1869 287 [21]. 289 o Editorial and clarification changes to RFC 2821 [29] to bring that 290 specification to Draft Standard. 292 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC 293 1123 (replacing the mail transport materials of RFC 1123). However, 294 RFC 821 specifies some features that were not in significant use in 295 the Internet by the mid-1990s and (in appendices) some additional 296 transport models. Those sections are omitted here in the interest of 297 clarity and brevity; readers needing them should refer to RFC 821. 299 It also includes some additional material from RFC 1123 that required 300 amplification. This material has been identified in multiple ways, 301 mostly by tracking flaming on various lists and newsgroups and 302 problems of unusual readings or interpretations that have appeared as 303 the SMTP extensions have been deployed. Where this specification 304 moves beyond consolidation and actually differs from earlier 305 documents, it supersedes them technically as well as textually. 307 Although SMTP was designed as a mail transport and delivery protocol, 308 this specification also contains information that is important to its 309 use as a "mail submission" protocol, as recommended for Post Office 310 Protocol (POP) (RFC 937 [13], RFC 1939 [22]) and IMAP (RFC 3501 311 [36]). In general, the separate mail submission protocol specified 312 in RFC 4409 [42] is now preferred to direct use of SMTP; more 313 discussion of that subject appears in that document. 315 Section 2.3 provides definitions of terms specific to this document. 316 Except when the historical terminology is necessary for clarity, this 317 document uses the current 'client' and 'server' terminology to 318 identify the sending and receiving SMTP processes, respectively. 320 A companion document, RFC 5322 [11], discusses message header 321 sections and bodies and specifies formats and structures for them. 323 1.3. Document Conventions 325 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 326 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 327 document are to be interpreted as described in RFC 2119 [1]. As each 328 of these terms was intentionally and carefully chosen to improve the 329 interoperability of email, each use of these terms is to be treated 330 as a conformance requirement. 332 Because this document has a long history and to avoid the risk of 333 various errors and of confusing readers and documents that point to 334 this one, most examples and the domain names they contain are 335 preserved from RFC 2821. Readers are cautioned that these are 336 illustrative examples that should not actually be used in either code 337 or configuration files. 339 2. The SMTP Model 341 [[CREF2: [5321bis] [[Editor's Note: There have been extensive and 342 repeated discussions on the SMTP and IETF lists about whether this 343 document should say something about hop-by-hop (MTA-to-MTA) SMTP 344 authentication and, if so, what?? Note that end to end message 345 authentication is almost certainly out of scope for SMTP.]]]] 347 2.1. Basic Structure 349 The SMTP design can be pictured as: 351 +----------+ +----------+ 352 +------+ | | | | 353 | User |<-->| | SMTP | | 354 +------+ | Client- |Commands/Replies| Server- | 355 +------+ | SMTP |<-------------->| SMTP | +------+ 356 | File |<-->| | and Mail | |<-->| File | 357 |System| | | | | |System| 358 +------+ +----------+ +----------+ +------+ 359 SMTP client SMTP server 361 When an SMTP client has a message to transmit, it establishes a two- 362 way transmission channel to an SMTP server. The responsibility of an 363 SMTP client is to transfer mail messages to one or more SMTP servers, 364 or report its failure to do so. 366 The means by which a mail message is presented to an SMTP client, and 367 how that client determines the identifier(s) ("names") of the 368 domain(s) to which mail messages are to be transferred, are local 369 matters. They are not addressed by this document. In some cases, 370 the designated domain(s), or those determined by an SMTP client, will 371 identify the final destination(s) of the mail message. In other 372 cases, common with SMTP clients associated with implementations of 373 the POP (RFC 937 [13], RFC 1939 [22]) or IMAP (RFC 3501 [36]) 374 protocols, or when the SMTP client is inside an isolated transport 375 service environment, the domain determined will identify an 376 intermediate destination through which all mail messages are to be 377 relayed. SMTP clients that transfer all traffic regardless of the 378 target domains associated with the individual messages, or that do 379 not maintain queues for retrying message transmissions that initially 380 cannot be completed, may otherwise conform to this specification but 381 are not considered fully-capable. Fully-capable SMTP 382 implementations, including the relays used by these less capable 383 ones, and their destinations, are expected to support all of the 384 queuing, retrying, and alternate address functions discussed in this 385 specification. In many situations and configurations, the less- 386 capable clients discussed above SHOULD be using the message 387 submission protocol (RFC 4409 [42]) rather than SMTP. 389 The means by which an SMTP client, once it has determined a target 390 domain, determines the identity of an SMTP server to which a copy of 391 a message is to be transferred, and then performs that transfer, are 392 covered by this document. To effect a mail transfer to an SMTP 393 server, an SMTP client establishes a two-way transmission channel to 394 that SMTP server. An SMTP client determines the address of an 395 appropriate host running an SMTP server by resolving a destination 396 domain name to either an intermediate Mail eXchanger host or a final 397 target host. 399 An SMTP server may be either the ultimate destination or an 400 intermediate "relay" (that is, it may assume the role of an SMTP 401 client after receiving the message) or "gateway" (that is, it may 402 transport the message further using some protocol other than SMTP). 403 SMTP commands are generated by the SMTP client and sent to the SMTP 404 server. SMTP replies are sent from the SMTP server to the SMTP 405 client in response to the commands. 407 In other words, message transfer can occur in a single connection 408 between the original SMTP-sender and the final SMTP-recipient, or can 409 occur in a series of hops through intermediary systems. In either 410 case, once the server has issued a success response at the end of the 411 mail data, a formal handoff of responsibility for the message occurs: 412 the protocol requires that a server MUST accept responsibility for 413 either delivering the message or properly reporting the failure to do 414 so (see Sections 6.1, 6.2, and 7.8, below). 416 Once the transmission channel is established and initial handshaking 417 is completed, the SMTP client normally initiates a mail transaction. 418 Such a transaction consists of a series of commands to specify the 419 originator and destination of the mail and transmission of the 420 message content (including any lines in the header section or other 421 structure) itself. When the same message is sent to multiple 422 recipients, this protocol encourages the transmission of only one 423 copy of the data for all recipients at the same destination (or 424 intermediate relay) host. 426 The server responds to each command with a reply; replies may 427 indicate that the command was accepted, that additional commands are 428 expected, or that a temporary or permanent error condition exists. 429 Commands specifying the sender or recipients may include server- 430 permitted SMTP service extension requests, as discussed in 431 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 432 although this can be modified by mutually agreed upon extension 433 requests such as command pipelining (RFC 2920 [30]). 435 Once a given mail message has been transmitted, the client may either 436 request that the connection be shut down or may initiate other mail 437 transactions. In addition, an SMTP client may use a connection to an 438 SMTP server for ancillary services such as verification of email 439 addresses or retrieval of mailing list subscriber addresses. 441 As suggested above, this protocol provides mechanisms for the 442 transmission of mail. Historically, this transmission normally 443 occurred directly from the sending user's host to the receiving 444 user's host when the two hosts are connected to the same transport 445 service. When they are not connected to the same transport service, 446 transmission occurs via one or more relay SMTP servers. A very 447 common case in the Internet today involves submission of the original 448 message to an intermediate, "message submission" server, which is 449 similar to a relay but has some additional properties; such servers 450 are discussed in Section 2.3.10 and at some length in RFC 4409 [42]. 451 An intermediate host that acts as either an SMTP relay or as a 452 gateway into some other transmission environment is usually selected 453 through the use of the domain name service (DNS) Mail eXchanger 454 mechanism. Explicit "source" routing (see Section 5 and Appendix C 455 and Appendix F.2) SHOULD NOT be used. [[CREF3: [5321bis] JcK 456 20090123 - redundant sentence removed.]] 458 2.2. The Extension Model 460 2.2.1. Background 462 In an effort that started in 1990, approximately a decade after RFC 463 821 was completed, the protocol was modified with a "service 464 extensions" model that permits the client and server to agree to 465 utilize shared functionality beyond the original SMTP requirements. 466 The SMTP extension mechanism defines a means whereby an extended SMTP 467 client and server may recognize each other, and the server can inform 468 the client as to the service extensions that it supports. 470 Contemporary SMTP implementations MUST support the basic extension 471 mechanisms. For instance, servers MUST support the EHLO command even 472 if they do not implement any specific extensions and clients SHOULD 473 preferentially utilize EHLO rather than HELO. (However, for 474 compatibility with older conforming implementations, SMTP clients and 475 servers MUST support the original HELO mechanisms as a fallback.) 476 Unless the different characteristics of HELO must be identified for 477 interoperability purposes, this document discusses only EHLO. 479 SMTP is widely deployed and high-quality implementations have proven 480 to be very robust. However, the Internet community now considers 481 some services to be important that were not anticipated when the 482 protocol was first designed. If support for those services is to be 483 added, it must be done in a way that permits older implementations to 484 continue working acceptably. The extension framework consists of: 486 o The SMTP command EHLO, superseding the earlier HELO, 488 o a registry of SMTP service extensions, 490 o additional parameters to the SMTP MAIL and RCPT commands, and 492 o optional replacements for commands defined in this protocol, such 493 as for DATA in non-ASCII transmissions (RFC 3030 [32]). 495 SMTP's strength comes primarily from its simplicity. Experience with 496 many protocols has shown that protocols with few options tend towards 497 ubiquity, whereas protocols with many options tend towards obscurity. 499 Each and every extension, regardless of its benefits, must be 500 carefully scrutinized with respect to its implementation, deployment, 501 and interoperability costs. In many cases, the cost of extending the 502 SMTP service will likely outweigh the benefit. 504 2.2.2. Definition and Registration of Extensions 506 The IANA maintains a registry of SMTP service extensions. A 507 corresponding EHLO keyword value is associated with each extension. 508 Each service extension registered with the IANA must be defined in a 509 formal Standards-Track or IESG-approved Experimental protocol 510 document. The definition must include: 512 o the textual name of the SMTP service extension; 514 o the EHLO keyword value associated with the extension; 516 o the syntax and possible values of parameters associated with the 517 EHLO keyword value; 519 o any additional SMTP verbs associated with the extension 520 (additional verbs will usually be, but are not required to be, the 521 same as the EHLO keyword value); 523 o any new parameters the extension associates with the MAIL or RCPT 524 verbs; 526 o a description of how support for the extension affects the 527 behavior of a server and client SMTP; and 529 o the increment by which the extension is increasing the maximum 530 length of the commands MAIL and/or RCPT, over that specified in 531 this Standard. 533 In addition, any EHLO keyword value starting with an upper or lower 534 case "X" refers to a local SMTP service extension used exclusively 535 through bilateral agreement. Keywords beginning with "X" MUST NOT be 536 used in a registered service extension. Conversely, keyword values 537 presented in the EHLO response that do not begin with "X" MUST 538 correspond to a Standard, Standards-Track, or IESG-approved 539 Experimental SMTP service extension registered with IANA. A 540 conforming server MUST NOT offer non-"X"-prefixed keyword values that 541 are not described in a registered extension. 543 Additional verbs and parameter names are bound by the same rules as 544 EHLO keywords; specifically, verbs beginning with "X" are local 545 extensions that may not be registered or standardized. Conversely, 546 verbs not beginning with "X" must always be registered. 548 2.2.3. Special Issues with Extensions 550 Extensions that change fairly basic properties of SMTP operation are 551 permitted. The text in other sections of this document must be 552 understood in that context. In particular, extensions can change the 553 minimum limits specified in Section 4.5.3, can change the ASCII 554 character set requirement as mentioned above, or can introduce some 555 optional modes of message handling. 557 In particular, if an extension implies that the delivery path 558 normally supports special features of that extension, and an 559 intermediate SMTP system finds a next hop that does not support the 560 required extension, it MAY choose, based on the specific extension 561 and circumstances, to requeue the message and try later and/or try an 562 alternate MX host. If this strategy is employed, the timeout to fall 563 back to an unextended format (if one is available) SHOULD be less 564 than the normal timeout for bouncing as undeliverable (e.g., if 565 normal timeout is three days, the requeue timeout before attempting 566 to transmit the mail without the extension might be one day). 568 2.3. SMTP Terminology 569 2.3.1. Mail Objects 571 SMTP transports a mail object. A mail object contains an envelope 572 and content. 574 The SMTP envelope is sent as a series of SMTP protocol units 575 (described in Section 3). It consists of an originator address (to 576 which error reports should be directed), one or more recipient 577 addresses, and optional protocol extension material. Historically, 578 variations on the reverse-path (originator) address specification 579 command (MAIL) could be used to specify alternate delivery modes, 580 such as immediate display; those variations have now been deprecated 581 (see Appendix F and Appendix F.6). 583 The SMTP content is sent in the SMTP DATA protocol unit and has two 584 parts: the header section and the body. If the content conforms to 585 other contemporary standards, the header section consists of a 586 collection of header fields, each consisting of a header name, a 587 colon, and data, structured as in the message format specification 588 (RFC 5322 [11]); the body, if structured, is defined according to 589 MIME (RFC 2045 [24]). The content is textual in nature, expressed 590 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 591 "8BITMIME", RFC 6152 [47]) may relax this restriction for the content 592 body, the content header fields are always encoded using the US-ASCII 593 repertoire. Two MIME extensions (RFC 2047 [25] and RFC 2231 [28]) 594 define an algorithm for representing header values outside the US- 595 ASCII repertoire, while still encoding them using the US-ASCII 596 repertoire. 598 2.3.2. Senders and Receivers 600 In RFC 821, the two hosts participating in an SMTP transaction were 601 described as the "SMTP-sender" and "SMTP-receiver". This document 602 has been changed to reflect current industry terminology and hence 603 refers to them as the "SMTP client" (or sometimes just "the client") 604 and "SMTP server" (or just "the server"), respectively. Since a 605 given host may act both as server and client in a relay situation, 606 "receiver" and "sender" terminology is still used where needed for 607 clarity. 609 2.3.3. Mail Agents and Message Stores 611 Additional mail system terminology became common after RFC 821 was 612 published and, where convenient, is used in this specification. In 613 particular, SMTP servers and clients provide a mail transport service 614 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 615 Agents" (MUAs or UAs) are normally thought of as the sources and 616 targets of mail. At the source, an MUA might collect mail to be 617 transmitted from a user and hand it off to an MTA; the final 618 ("delivery") MTA would be thought of as handing the mail off to an 619 MUA (or at least transferring responsibility to it, e.g., by 620 depositing the message in a "message store"). However, while these 621 terms are used with at least the appearance of great precision in 622 other environments, the implied boundaries between MUAs and MTAs 623 often do not accurately match common, and conforming, practices with 624 Internet mail. Hence, the reader should be cautious about inferring 625 the strong relationships and responsibilities that might be implied 626 if these terms were used elsewhere. 628 2.3.4. Host 630 For the purposes of this specification, a host is a computer system 631 attached to the Internet (or, in some cases, to a private TCP/IP 632 network) and supporting the SMTP protocol. Hosts are known by names 633 (see the next section); they SHOULD NOT be identified by numerical 634 addresses, i.e., by address literals as described in Section 4.1.2. 636 2.3.5. Domain Names 638 A domain name (or often just a "domain") consists of one or more 639 components, separated by dots if more than one appears. In the case 640 of a top-level domain used by itself in an email address, a single 641 string is used without any dots. This makes the requirement, 642 described in more detail below, that only fully-qualified domain 643 names appear in SMTP transactions on the public Internet, 644 particularly important where top-level domains are involved. These 645 components ("labels" in DNS terminology, RFC 1035 [4]) are restricted 646 for SMTP purposes to consist of a sequence of letters, digits, and 647 hyphens drawn from the ASCII character set [2] and conforming to what 648 RFC 1035 Section 2.3.1 calls the "preferred name syntax". Domain 649 names are used as names of hosts and of other entities in the domain 650 name hierarchy. For example, a domain may refer to an alias (label 651 of a CNAME RR) or the label of Mail eXchanger records to be used to 652 deliver mail instead of representing a host name. See RFC 1035 [4] 653 and Section 5 of this specification. 655 The domain name, as described in this document and in RFC 1035 [4], 656 is the entire, fully-qualified name (often referred to as an "FQDN"). 657 A domain name that is not in FQDN form is no more than a local alias. 658 Local aliases MUST NOT appear in any SMTP transaction. 660 Only resolvable, fully-qualified domain names (FQDNs) are permitted 661 when domain names are used in SMTP. 662 [[CREF4: [[5321bis Editor's Note: does "in the public DNS" or 663 equivalent need to be added to "resolvable"???]]]] 664 In other words, names that can be resolved to MX RRs or address 665 (i.e., A or AAAA) RRs (as discussed in Section 5) are permitted, as 666 are CNAME RRs whose targets can be resolved, in turn, to MX or 667 address RRs. 668 [[CREF5: [[5321bis Editor's Note: it is not clear whether "In other 669 words" really meant "for example" or it is was intended that the only 670 labels permitted are those that own records in one of the above RR 671 types]]]] 672 [[CREF6: [[5321bis Editor's Note: More generally, does this section 673 need work to clarify the relationship to private domain names 674 (discussed on SMTP list starting 2013-03-26)]]]] 675 Local nicknames or unqualified names MUST NOT be used. There are two 676 exceptions to the rule requiring FQDNs: 678 o The domain name given in the EHLO command MUST be either a primary 679 host name (a domain name that resolves to an address RR) or, if 680 the host has no name, an address literal, as described in 681 Section 4.1.3 and discussed further in the EHLO discussion of 682 Section 4.1.4. 684 o The reserved mailbox name "postmaster" may be used in a RCPT 685 command without domain qualification (see Section 4.1.1.3) and 686 MUST be accepted if so used. 688 2.3.6. Buffer and State Table 690 SMTP sessions are stateful, with both parties carefully maintaining a 691 common view of the current state. In this document, we model this 692 state by a virtual "buffer" and a "state table" on the server that 693 may be used by the client to, for example, "clear the buffer" or 694 "reset the state table", causing the information in the buffer to be 695 discarded and the state to be returned to some previous state. 697 2.3.7. Commands and Replies 699 SMTP commands and, unless altered by a service extension, message 700 data, are transmitted from the sender to the receiver via the 701 transmission channel in "lines". 703 An SMTP reply is an acknowledgment (positive or negative) sent in 704 "lines" from receiver to sender via the transmission channel in 705 response to a command. The general form of a reply is a numeric 706 completion code (indicating failure or success) usually followed by a 707 text string. The codes are for use by programs and the text is 708 usually intended for human users. RFC 3463 [34], specifies further 709 structuring of the reply strings, including the use of supplemental 710 and more specific completion codes (see also RFC 5248 [46]). 712 2.3.8. Lines 714 Lines consist of zero or more data characters terminated by the 715 sequence ASCII character "CR" (hex value 0D) followed immediately by 716 ASCII character "LF" (hex value 0A). This termination sequence is 717 denoted as in this document. Conforming implementations MUST 718 NOT recognize or generate any other character or character sequence 719 as a line terminator. Limits MAY be imposed on line lengths by 720 servers (see Section 4). 722 In addition, the appearance of "bare" "CR" or "LF" characters in text 723 (i.e., either without the other) has a long history of causing 724 problems in mail implementations and applications that use the mail 725 system as a tool. SMTP client implementations MUST NOT transmit 726 these characters except when they are intended as line terminators 727 and then MUST, as indicated above, transmit them only as a 728 sequence. 730 2.3.9. Message Content and Mail Data 732 The terms "message content" and "mail data" are used interchangeably 733 in this document to describe the material transmitted after the DATA 734 command is accepted and before the end of data indication is 735 transmitted. Message content includes the message header section and 736 the possibly structured message body. The MIME specification (RFC 737 2045 [24]) provides the standard mechanisms for structured message 738 bodies. 740 2.3.10. Originator, Delivery, Relay, and Gateway Systems 742 This specification makes a distinction among four types of SMTP 743 systems, based on the role those systems play in transmitting 744 electronic mail. An "originating" system (sometimes called an SMTP 745 originator) introduces mail into the Internet or, more generally, 746 into a transport service environment. A "delivery" SMTP system is 747 one that receives mail from a transport service environment and 748 passes it to a mail user agent or deposits it in a message store that 749 a mail user agent is expected to subsequently access. A "relay" SMTP 750 system (usually referred to just as a "relay") receives mail from an 751 SMTP client and transmits it, without modification to the message 752 data other than adding trace information, to another SMTP server for 753 further relaying or for delivery. 755 A "gateway" SMTP system (usually referred to just as a "gateway") 756 receives mail from a client system in one transport environment and 757 transmits it to a server system in another transport environment. 758 Differences in protocols or message semantics between the transport 759 environments on either side of a gateway may require that the gateway 760 system perform transformations to the message that are not permitted 761 to SMTP relay systems. For the purposes of this specification, 762 firewalls that rewrite addresses should be considered as gateways, 763 even if SMTP is used on both sides of them (see RFC 2979 [31]). 764 [[CREF7: [5321bis] [[Note in draft/Placeholder: There has been a 765 request to expand this section, possibly into a more extensive model 766 of Internet mail. Comments from others solicited. In particular, 767 does RFC 5598 make that suggestion OBE?]] ]] 769 2.3.11. Mailbox and Address 771 As used in this specification, an "address" is a character string 772 that identifies a user to whom mail will be sent or a location into 773 which mail will be deposited. The term "mailbox" refers to that 774 depository. The two terms are typically used interchangeably unless 775 the distinction between the location in which mail is placed (the 776 mailbox) and a reference to it (the address) is important. An 777 address normally consists of user and domain specifications. The 778 standard mailbox naming convention is defined to be "local- 779 part@domain"; contemporary usage permits a much broader set of 780 applications than simple "user names". Consequently, and due to a 781 long history of problems when intermediate hosts have attempted to 782 optimize transport by modifying them, the local-part MUST be 783 interpreted and assigned semantics only by the host specified in the 784 domain part of the address. 786 2.4. General Syntax Principles and Transaction Model 788 SMTP commands and replies have a rigid syntax. All commands begin 789 with a command verb. All replies begin with a three digit numeric 790 code. In some commands and replies, arguments are required following 791 the verb or reply code. Some commands do not accept arguments (after 792 the verb), and some reply codes are followed, sometimes optionally, 793 by free form text. In both cases, where text appears, it is 794 separated from the verb or reply code by a space character. Complete 795 definitions of commands and replies appear in Section 4. 797 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 798 and extension name keywords) are not case sensitive, with the sole 799 exception in this specification of a mailbox local-part (SMTP 800 Extensions may explicitly specify case-sensitive elements). That is, 801 a command verb, an argument value other than a mailbox local-part, 802 and free form text MAY be encoded in upper case, lower case, or any 803 mixture of upper and lower case with no impact on its meaning. The 804 local-part of a mailbox MUST BE treated as case sensitive. 805 Therefore, SMTP implementations MUST take care to preserve the case 806 of mailbox local-parts. In particular, for some hosts, the user 807 "smith" is different from the user "Smith". However, exploiting the 808 case sensitivity of mailbox local-parts impedes interoperability and 809 is discouraged. Mailbox domains follow normal DNS rules and are 810 hence not case sensitive. 812 A few SMTP servers, in violation of this specification (and RFC 821) 813 require that command verbs be encoded by clients in upper case. 814 Implementations MAY wish to employ this encoding to accommodate those 815 servers. 817 The argument clause consists of a variable-length character string 818 ending with the end of the line, i.e., with the character sequence 819 . The receiver will take no action until this sequence is 820 received. 822 The syntax for each command is shown with the discussion of that 823 command. Common elements and parameters are shown in Section 4.1.2. 825 Commands and replies are composed of characters from the ASCII 826 character set [2]. When the transport service provides an 8-bit byte 827 (octet) transmission channel, each 7-bit character is transmitted, 828 right justified, in an octet with the high-order bit cleared to zero. 829 More specifically, the unextended SMTP service provides 7-bit 830 transport only. An originating SMTP client that has not successfully 831 negotiated an appropriate extension with a particular server (see the 832 next paragraph) MUST NOT transmit messages with information in the 833 high-order bit of octets. If such messages are transmitted in 834 violation of this rule, receiving SMTP servers MAY clear the high- 835 order bit or reject the message as invalid. In general, a relay SMTP 836 SHOULD assume that the message content it has received is valid and, 837 assuming that the envelope permits doing so, relay it without 838 inspecting that content. Of course, if the content is mislabeled and 839 the data path cannot accept the actual content, this may result in 840 the ultimate delivery of a severely garbled message to the recipient. 841 Delivery SMTP systems MAY reject such messages, or return them as 842 undeliverable, rather than deliver them. In the absence of a server- 843 offered extension explicitly permitting it, a sending SMTP system is 844 not permitted to send envelope commands in any character set other 845 than US-ASCII. Receiving systems SHOULD reject such commands, 846 normally using "500 syntax error - invalid character" replies. 848 8-bit message content transmission MAY be requested of the server by 849 a client using extended SMTP facilities, notably the "8BITMIME" 850 extension, RFC 6152 [47]. 8BITMIME SHOULD be supported by SMTP 851 servers. However, it MUST NOT be construed as authorization to 852 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 853 transmission of any envelope material in other than ASCII. 8BITMIME 854 MUST NOT be requested by senders for material with the high bit on 855 that is not in MIME format with an appropriate content-transfer 856 encoding; servers MAY reject such messages. 858 The metalinguistic notation used in this document corresponds to the 859 "Augmented BNF" used in other Internet mail system documents. The 860 reader who is not familiar with that syntax should consult the ABNF 861 specification in RFC 5234 [10]. Metalanguage terms used in running 862 text are surrounded by pointed brackets (e.g., ) for clarity. 863 The reader is cautioned that the grammar expressed in the 864 metalanguage is not comprehensive. There are many instances in which 865 provisions in the text constrain or otherwise modify the syntax or 866 semantics implied by the grammar. 868 3. The SMTP Procedures: An Overview 870 This section contains descriptions of the procedures used in SMTP: 871 session initiation, mail transaction, forwarding mail, verifying 872 mailbox names and expanding mailing lists, and opening and closing 873 exchanges. Comments on relaying, a note on mail domains, and a 874 discussion of changing roles are included at the end of this section. 875 Several complete scenarios are presented in Appendix D. 877 3.1. Session Initiation 879 An SMTP session is initiated when a client opens a connection to a 880 server and the server responds with an opening message. 882 SMTP server implementations MAY include identification of their 883 software and version information in the connection greeting reply 884 after the 220 code, a practice that permits more efficient isolation 885 and repair of any problems. Implementations MAY make provision for 886 SMTP servers to disable the software and version announcement where 887 it causes security concerns. While some systems also identify their 888 contact point for mail problems, this is not a substitute for 889 maintaining the required "postmaster" address (see Section 4). 891 The SMTP protocol allows a server to formally reject a mail session 892 while still allowing the initial connection as follows: a 554 893 response MAY be given in the initial connection opening message 894 instead of the 220. A server taking this approach MUST still wait 895 for the client to send a QUIT (see Section 4.1.1.10) before closing 896 the connection and SHOULD respond to any intervening commands with 897 "503 bad sequence of commands". Since an attempt to make an SMTP 898 connection to such a system is probably in error, a server returning 899 a 554 response on connection opening SHOULD provide enough 900 information in the reply text to facilitate debugging of the sending 901 system. 903 3.2. Client Initiation 905 Once the server has sent the greeting (welcoming) message and the 906 client has received it, the client normally sends the EHLO command to 907 the server, indicating the client's identity. In addition to opening 908 the session, use of EHLO indicates that the client is able to process 909 service extensions and requests that the server provide a list of the 910 extensions it supports. Older SMTP systems that are unable to 911 support service extensions, and contemporary clients that do not 912 require service extensions in the mail session being initiated, MAY 913 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 914 style response to a HELO command. For a particular connection 915 attempt, if the server returns a "command not recognized" response to 916 EHLO, the client SHOULD be able to fall back and send HELO. 918 In the EHLO command, the host sending the command identifies itself; 919 the command may be interpreted as saying "Hello, I am " (and, 920 in the case of EHLO, "and I support service extension requests"). 922 3.3. Mail Transactions 924 There are three steps to SMTP mail transactions. The transaction 925 starts with a MAIL command that gives the sender identification. (In 926 general, the MAIL command may be sent only when no mail transaction 927 is in progress; see Section 4.1.4.) A series of one or more RCPT 928 commands follows, giving the receiver information. Then, a DATA 929 command initiates transfer of the mail data and is terminated by the 930 "end of mail" data indicator, which also confirms the transaction. 932 The first step in the procedure is the MAIL command. 934 MAIL FROM: [SP ] 936 This command tells the SMTP-receiver that a new mail transaction is 937 starting and to reset all its state tables and buffers, including any 938 recipients or mail data. The portion of the first or 939 only argument contains the source mailbox (between "<" and ">" 940 brackets), which can be used to report errors (see Section 4.2 for a 941 discussion of error reporting). If accepted, the SMTP server returns 942 a "250 OK" reply. If the mailbox specification is not acceptable for 943 some reason, the server MUST return a reply indicating whether the 944 failure is permanent (i.e., will occur again if the client tries to 945 send the same address again) or temporary (i.e., the address might be 946 accepted if the client tries again later). Despite the apparent 947 scope of this requirement, there are circumstances in which the 948 acceptability of the reverse-path may not be determined until one or 949 more forward-paths (in RCPT commands) can be examined. In those 950 cases, the server MAY reasonably accept the reverse-path (with a 250 951 reply) and then report problems after the forward-paths are received 952 and examined. Normally, failures produce 550 or 553 replies. 954 Historically, the was permitted to contain more than 955 just a mailbox; however, contemporary systems SHOULD NOT use source 956 routing (see Appendix C). 958 The optional are associated with negotiated SMTP 959 service extensions (see Section 2.2). 961 The second step in the procedure is the RCPT command. This step of 962 the procedure can be repeated any number of times. 964 RCPT TO: [ SP ] 966 The first or only argument to this command includes a forward-path 967 (normally a mailbox and domain, always surrounded by "<" and ">" 968 brackets) identifying one recipient. If accepted, the SMTP server 969 returns a "250 OK" reply and stores the forward-path. If the 970 recipient is known not to be a deliverable address, the SMTP server 971 returns a 550 reply, typically with a string such as "no such user - 972 " and the mailbox name (other circumstances and reply codes are 973 possible). 975 The can contain more than just a mailbox. 976 Historically, the was permitted to contain a source 977 routing list of hosts and the destination mailbox; however, 978 contemporary SMTP clients SHOULD NOT utilize source routes (see 979 Appendix C). Servers MUST be prepared to encounter a list of source 980 routes in the forward-path, but they SHOULD ignore the routes or MAY 981 decline to support the relaying they imply. Similarly, servers MAY 982 decline to accept mail that is destined for other hosts or systems. 983 These restrictions make a server useless as a relay for clients that 984 do not support full SMTP functionality. Consequently, restricted- 985 capability clients MUST NOT assume that any SMTP server on the 986 Internet can be used as their mail processing (relaying) site. If a 987 RCPT command appears without a previous MAIL command, the server MUST 988 return a 503 "Bad sequence of commands" response. The optional 989 are associated with negotiated SMTP service 990 extensions (see Section 2.2). [[CREF8: [5321bis] JcK Note for 991 2821ter (5321bis): this section would be improved by being more 992 specific about where mail transactions begin and end and then talking 993 about "transaction state" here, rather than specific prior commands. 994 --JcK]] 996 Since it has been a common source of errors, it is worth noting that 997 spaces are not permitted on either side of the colon following FROM 998 in the MAIL command or TO in the RCPT command. The syntax is exactly 999 as given above. 1001 The third step in the procedure is the DATA command (or some 1002 alternative specified in a service extension). 1004 DATA 1006 If accepted, the SMTP server returns a 354 Intermediate reply and 1007 considers all succeeding lines up to but not including the end of 1008 mail data indicator to be the message text. When the end of text is 1009 successfully received and stored, the SMTP-receiver sends a "250 OK" 1010 reply. 1012 Since the mail data is sent on the transmission channel, the end of 1013 mail data must be indicated so that the command and reply dialog can 1014 be resumed. SMTP indicates the end of the mail data by sending a 1015 line containing only a "." (period or full stop). A transparency 1016 procedure is used to prevent this from interfering with the user's 1017 text (see Section 4.5.2). 1019 The end of mail data indicator also confirms the mail transaction and 1020 tells the SMTP server to now process the stored recipients and mail 1021 data. If accepted, the SMTP server returns a "250 OK" reply. The 1022 DATA command can fail at only two points in the protocol exchange: 1024 If there was no MAIL, or no RCPT, command, or all such commands were 1025 rejected, the server MAY return a "command out of sequence" (503) or 1026 "no valid recipients" (554) reply in response to the DATA command. 1027 If one of those replies (or any other 5yz reply) is received, the 1028 client MUST NOT send the message data; more generally, message data 1029 MUST NOT be sent unless a 354 reply is received. 1031 If the verb is initially accepted and the 354 reply issued, the DATA 1032 command should fail only if the mail transaction was incomplete (for 1033 example, no recipients), if resources were unavailable (including, of 1034 course, the server unexpectedly becoming unavailable), or if the 1035 server determines that the message should be rejected for policy or 1036 other reasons. 1038 However, in practice, some servers do not perform recipient 1039 verification until after the message text is received. These servers 1040 SHOULD treat a failure for one or more recipients as a "subsequent 1041 failure" and return a mail message as discussed in Section 6 and, in 1042 particular, in Section 6.1. Using a "550 mailbox not found" (or 1043 equivalent) reply code after the data are accepted makes it difficult 1044 or impossible for the client to determine which recipients failed. 1046 When the RFC 822 format ([12], [11]) is being used, the mail data 1047 include the header fields such as those named Date, Subject, To, Cc, 1048 and From. Server SMTP systems SHOULD NOT reject messages based on 1049 perceived defects in the RFC 822 or MIME (RFC 2045 [24]) message 1050 header section or message body. In particular, they MUST NOT reject 1051 messages in which the numbers of Resent-header fields do not match or 1052 Resent-to appears without Resent-from and/or Resent-date. 1054 Mail transaction commands MUST be used in the order discussed above. 1056 3.4. Forwarding for Address Correction or Updating 1058 Forwarding support is most often required to consolidate and simplify 1059 addresses within, or relative to, some enterprise and less frequently 1060 to establish addresses to link a person's prior address with a 1061 current one. Silent forwarding of messages (without server 1062 notification to the sender), for security or non-disclosure purposes, 1063 is common in the contemporary Internet. 1065 In both the enterprise and the "new address" cases, information 1066 hiding (and sometimes security) considerations argue against exposure 1067 of the "final" address through the SMTP protocol as a side effect of 1068 the forwarding activity. This may be especially important when the 1069 final address may not even be reachable by the sender. Consequently, 1070 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1071 especially the 251 (corrected destination) and 551 reply codes from 1072 RCPT must be evaluated carefully by implementers and, when they are 1073 available, by those configuring systems (see also Section 7.4). 1075 In particular: 1077 o Servers MAY forward messages when they are aware of an address 1078 change. When they do so, they MAY either provide address-updating 1079 information with a 251 code, or may forward "silently" and return 1080 a 250 code. However, if a 251 code is used, they MUST NOT assume 1081 that the client will actually update address information or even 1082 return that information to the user. 1084 Alternately, 1086 o Servers MAY reject messages or return them as non-deliverable when 1087 they cannot be delivered precisely as addressed. When they do so, 1088 they MAY either provide address-updating information with a 551 1089 code, or may reject the message as undeliverable with a 550 code 1090 and no address-specific information. However, if a 551 code is 1091 used, they MUST NOT assume that the client will actually update 1092 address information or even return that information to the user. 1094 SMTP server implementations that support the 251 and/or 551 reply 1095 codes SHOULD provide configuration mechanisms so that sites that 1096 conclude that they would undesirably disclose information can disable 1097 or restrict their use. 1099 3.5. Commands for Debugging Addresses 1101 3.5.1. Overview 1103 SMTP provides commands to verify a user name or obtain the content of 1104 a mailing list. This is done with the VRFY and EXPN commands, which 1105 have character string arguments. Implementations SHOULD support VRFY 1106 and EXPN (however, see Section 3.5.2 and Section 7.3). 1108 For the VRFY command, the string is a user name or a user name and 1109 domain (see below). If a normal (i.e., 250) response is returned, 1110 the response MAY include the full name of the user and MUST include 1111 the mailbox of the user. It MUST be in either of the following 1112 forms: 1114 User Name 1115 local-part@domain 1117 When a name that is the argument to VRFY could identify more than one 1118 mailbox, the server MAY either note the ambiguity or identify the 1119 alternatives. In other words, any of the following are legitimate 1120 responses to VRFY: 1122 553 User ambiguous 1124 or 1126 553- Ambiguous; Possibilities are 1127 553-Joe Smith 1128 553-Harry Smith 1129 553 Melvin Smith 1131 or 1133 553-Ambiguous; Possibilities 1134 553- 1135 553- 1136 553 1138 Under normal circumstances, a client receiving a 553 reply would be 1139 expected to expose the result to the user. Use of exactly the forms 1140 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1141 supplemented by extended reply codes, such as those described in RFC 1142 3463 [34], will facilitate automated translation into other languages 1143 as needed. Of course, a client that was highly automated or that was 1144 operating in another language than English might choose to try to 1145 translate the response to return some other indication to the user 1146 than the literal text of the reply, or to take some automated action 1147 such as consulting a directory service for additional information 1148 before reporting to the user. 1150 For the EXPN command, the string identifies a mailing list, and the 1151 successful (i.e., 250) multiline response MAY include the full name 1152 of the users and MUST give the mailboxes on the mailing list. 1154 In some hosts, the distinction between a mailing list and an alias 1155 for a single mailbox is a bit fuzzy, since a common data structure 1156 may hold both types of entries, and it is possible to have mailing 1157 lists containing only one mailbox. If a request is made to apply 1158 VRFY to a mailing list, a positive response MAY be given if a message 1159 so addressed would be delivered to everyone on the list, otherwise an 1160 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1161 user" or "252 Unable to verify members of mailing list"). If a 1162 request is made to expand a user name, the server MAY return a 1163 positive response consisting of a list containing one name, or an 1164 error MAY be reported (e.g., "550 That is a user name, not a mailing 1165 list"). 1167 In the case of a successful multiline reply (normal for EXPN), 1168 exactly one mailbox is to be specified on each line of the reply. 1169 The case of an ambiguous request is discussed above. 1171 "User name" is a fuzzy term and has been used deliberately. An 1172 implementation of the VRFY or EXPN commands MUST include at least 1173 recognition of local mailboxes as "user names". However, since 1174 current Internet practice often results in a single host handling 1175 mail for multiple domains, hosts, especially hosts that provide this 1176 functionality, SHOULD accept the "local-part@domain" form as a "user 1177 name"; hosts MAY also choose to recognize other strings as "user 1178 names". 1180 The case of expanding a mailbox list requires a multiline reply, such 1181 as: 1183 C: EXPN Example-People 1184 S: 250-Jon Postel 1185 S: 250-Fred Fonebone 1186 S: 250 Sam Q. Smith 1188 or 1189 C: EXPN Executive-Washroom-List 1190 S: 550 Access Denied to You. 1192 The character string arguments of the VRFY and EXPN commands cannot 1193 be further restricted due to the variety of implementations of the 1194 user name and mailbox list concepts. On some systems, it may be 1195 appropriate for the argument of the EXPN command to be a file name 1196 for a file containing a mailing list, but again there are a variety 1197 of file naming conventions in the Internet. Similarly, historical 1198 variations in what is returned by these commands are such that the 1199 response SHOULD be interpreted very carefully, if at all, and SHOULD 1200 generally only be used for diagnostic purposes. 1202 3.5.2. VRFY Normal Response 1204 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1205 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1207 domain name. In circumstances exceptional enough to justify 1208 violating the intent of this specification, free-form text MAY be 1209 returned. In order to facilitate parsing by both computers and 1210 people, addresses SHOULD appear in pointed brackets. When addresses, 1211 rather than free-form debugging information, are returned, EXPN and 1212 VRFY MUST return only valid domain addresses that are usable in SMTP 1213 RCPT commands. Consequently, if an address implies delivery to a 1214 program or other system, the mailbox name used to reach that target 1215 MUST be given. Paths (explicit source routes) MUST NOT be returned 1216 by VRFY or EXPN. 1218 Server implementations SHOULD support both VRFY and EXPN. For 1219 security reasons, implementations MAY provide local installations a 1220 way to disable either or both of these commands through configuration 1221 options or the equivalent (see Section 7.3). When these commands are 1222 supported, they are not required to work across relays when relaying 1223 is supported. Since they were both optional in RFC 821, but VRFY was 1224 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1225 listed as a service extension in an EHLO response. VRFY MAY be 1226 listed as a convenience but, since support for it is required, SMTP 1227 clients are not required to check for its presence on the extension 1228 list before using it. 1230 3.5.3. Meaning of VRFY or EXPN Success Response 1232 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1233 command unless it has actually verified the address. In particular, 1234 a server MUST NOT return 250 if all it has done is to verify that the 1235 syntax given is valid. In that case, 502 (Command not implemented) 1236 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1237 stated elsewhere, implementation (in the sense of actually validating 1238 addresses and returning information) of VRFY and EXPN are strongly 1239 recommended. Hence, implementations that return 500 or 502 for VRFY 1240 are not in full compliance with this specification. 1242 There may be circumstances where an address appears to be valid but 1243 cannot reasonably be verified in real time, particularly when a 1244 server is acting as a mail exchanger for another server or domain. 1245 "Apparent validity", in this case, would normally involve at least 1246 syntax checking and might involve verification that any domains 1247 specified were ones to which the host expected to be able to relay 1248 mail. In these situations, reply code 252 SHOULD be returned. These 1249 cases parallel the discussion of RCPT verification in Section 2.1. 1250 Similarly, the discussion in Section 3.4 applies to the use of reply 1251 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are 1252 recognized but that would be forwarded or rejected were mail received 1253 for them. Implementations generally SHOULD be more aggressive about 1254 address verification in the case of VRFY than in the case of RCPT, 1255 even if it takes a little longer to do so. 1257 3.5.4. Semantics and Applications of EXPN 1259 EXPN is often very useful in debugging and understanding problems 1260 with mailing lists and multiple-target-address aliases. Some systems 1261 have attempted to use source expansion of mailing lists as a means of 1262 eliminating duplicates. The propagation of aliasing systems with 1263 mail on the Internet for hosts (typically with MX and CNAME DNS 1264 records), for mailboxes (various types of local host aliases), and in 1265 various proxying arrangements has made it nearly impossible for these 1266 strategies to work consistently, and mail systems SHOULD NOT attempt 1267 them. 1269 3.6. Relaying and Mail Routing 1271 3.6.1. Source Routes and Relaying 1273 In general, the availability of Mail eXchanger records in the domain 1274 name system (RFC 1035 [4], RFC 974 [15]) makes the use of explicit 1275 source routes in the Internet mail system unnecessary. Many 1276 historical problems with the interpretation of explicit source routes 1277 have made their use undesirable. SMTP clients SHOULD NOT generate 1278 explicit source routes except under unusual circumstances. SMTP 1279 servers MAY decline to act as mail relays or to accept addresses that 1280 specify source routes. When route information is encountered, SMTP 1281 servers MAY ignore the route information and simply send to the final 1282 destination specified as the last element in the route and SHOULD do 1283 so. There has been an invalid practice of using names that do not 1284 appear in the DNS as destination names, with the senders counting on 1285 the intermediate hosts specified in source routing to resolve any 1286 problems. If source routes are stripped, this practice will cause 1287 failures. This is one of several reasons why SMTP clients MUST NOT 1288 generate invalid source routes or depend on serial resolution of 1289 names in such routes. [[CREF9: [5321bis] Jck 20091023: "of names..." 1290 added for clarity"]] 1292 When source routes are not used, the process described in RFC 821 for 1293 constructing a reverse-path from the forward-path is not applicable 1294 and the reverse-path at the time of delivery will simply be the 1295 address that appeared in the MAIL command. 1297 3.6.2. Mail eXchange Records and Relaying 1299 A relay SMTP server is usually the target of a DNS MX record that 1300 designates it, rather than the final delivery system. The relay 1301 server may accept or reject the task of relaying the mail in the same 1302 way it accepts or rejects mail for a local user. If it accepts the 1303 task, it then becomes an SMTP client, establishes a transmission 1304 channel to the next SMTP server specified in the DNS (according to 1305 the rules in Section 5), and sends it the mail. If it declines to 1306 relay mail to a particular address for policy reasons, a 550 response 1307 SHOULD be returned. 1309 This specification does not deal with the verification of return 1310 paths for use in delivery notifications. Recent work, such as that 1311 on SPF [41] and DKIM [43] [44], has been done to provide ways to 1312 ascertain that an address is valid or belongs to the person who 1313 actually sent the message. 1314 [[5321bis Editor's Note: Proposed erratum (4055) suggests that DKIM 1315 and SPF have nothing to do with this and that everything after the 1316 first sentence should be dropped. An alternative would be to tune 1317 the texts. ???]] 1318 A server MAY attempt to verify the return path before using its 1319 address for delivery notifications, but methods of doing so are not 1320 defined here nor is any particular method recommended at this time. 1322 3.6.3. Message Submission Servers as Relays 1324 Many mail-sending clients exist, especially in conjunction with 1325 facilities that receive mail via POP3 or IMAP, that have limited 1326 capability to support some of the requirements of this specification, 1327 such as the ability to queue messages for subsequent delivery 1328 attempts. For these clients, it is common practice to make private 1329 arrangements to send all messages to a single server for processing 1330 and subsequent distribution. SMTP, as specified here, is not ideally 1331 suited for this role. A standardized mail submission protocol has 1332 been developed that is gradually superseding practices based on SMTP 1333 (see RFC 4409 [42]). In any event, because these arrangements are 1334 private and fall outside the scope of this specification, they are 1335 not described here. 1337 It is important to note that MX records can point to SMTP servers 1338 that act as gateways into other environments, not just SMTP relays 1339 and final delivery systems; see Sections 3.7 and 5. 1341 If an SMTP server has accepted the task of relaying the mail and 1342 later finds that the destination is incorrect or that the mail cannot 1343 be delivered for some other reason, then it MUST construct an 1344 "undeliverable mail" notification message and send it to the 1345 originator of the undeliverable mail (as indicated by the reverse- 1346 path). Formats specified for non-delivery reports by other standards 1347 (see, for example, RFC 3461 [33] and RFC 3464 [35]) SHOULD be used if 1348 possible. 1350 This notification message must be from the SMTP server at the relay 1351 host or the host that first determines that delivery cannot be 1352 accomplished. Of course, SMTP servers MUST NOT send notification 1353 messages about problems transporting notification messages. One way 1354 to prevent loops in error reporting is to specify a null reverse-path 1355 in the MAIL command of a notification message. When such a message 1356 is transmitted, the reverse-path MUST be set to null (see 1357 Section 4.5.5 for additional discussion). A MAIL command with a null 1358 reverse-path appears as follows: 1360 MAIL FROM:<> 1362 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1363 act upon the header section or body of the message data and MUST NOT 1364 do so except to add its own "Received:" header field (Section 4.4) 1365 and, optionally, to attempt to detect looping in the mail system (see 1366 Section 6.3). Of course, this prohibition also applies to any 1367 modifications of these header fields or text (see also Section 7.9). 1369 3.7. Mail Gatewaying 1371 While the relay function discussed above operates within the Internet 1372 SMTP transport service environment, MX records or various forms of 1373 explicit routing may require that an intermediate SMTP server perform 1374 a translation function between one transport service and another. As 1375 discussed in Section 2.3.10, when such a system is at the boundary 1376 between two transport service environments, we refer to it as a 1377 "gateway" or "gateway SMTP". 1379 Gatewaying mail between different mail environments, such as 1380 different mail formats and protocols, is complex and does not easily 1381 yield to standardization. However, some general requirements may be 1382 given for a gateway between the Internet and another mail 1383 environment. 1385 3.7.1. Header Fields in Gatewaying 1387 Header fields MAY be rewritten when necessary as messages are 1388 gatewayed across mail environment boundaries. This may involve 1389 inspecting the message body or interpreting the local-part of the 1390 destination address in spite of the prohibitions in Section 6.4. 1392 Other mail systems gatewayed to the Internet often use a subset of 1393 the RFC 822 header section or provide similar functionality with a 1394 different syntax, but some of these mail systems do not have an 1395 equivalent to the SMTP envelope. Therefore, when a message leaves 1396 the Internet environment, it may be necessary to fold the SMTP 1397 envelope information into the message header section. A possible 1398 solution would be to create new header fields to carry the envelope 1399 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1400 would require changes in mail programs in foreign environments and 1401 might risk disclosure of private information (see Section 7.2). 1403 3.7.2. Received Lines in Gatewaying 1405 When forwarding a message into or out of the Internet environment, a 1406 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1407 way a Received: line that is already in the header section. 1409 "Received:" header fields of messages originating from other 1410 environments may not conform exactly to this specification. However, 1411 the most important use of Received: lines is for debugging mail 1412 faults, and this debugging can be severely hampered by well-meaning 1413 gateways that try to "fix" a Received: line. As another consequence 1414 of trace header fields arising in non-SMTP environments, receiving 1415 systems MUST NOT reject mail based on the format of a trace header 1416 field and SHOULD be extremely robust in the light of unexpected 1417 information or formats in those header fields. 1419 The gateway SHOULD indicate the environment and protocol in the "via" 1420 clauses of Received header field(s) that it supplies. 1422 3.7.3. Addresses in Gatewaying 1424 From the Internet side, the gateway SHOULD accept all valid address 1425 formats in SMTP commands and in the RFC 822 header section, and all 1426 valid RFC 822 messages. Addresses and header fields generated by 1427 gateways MUST conform to applicable standards (including this one and 1428 RFC 5322 [11]). Gateways are, of course, subject to the same rules 1429 for handling source routes as those described for other SMTP systems 1430 in Section 3.3. 1432 3.7.4. Other Header Fields in Gatewaying 1434 The gateway MUST ensure that all header fields of a message that it 1435 forwards into the Internet mail environment meet the requirements for 1436 Internet mail. In particular, all addresses in "From:", "To:", 1437 "Cc:", etc., header fields MUST be transformed (if necessary) to 1438 satisfy the standard header syntax of RFC 5322 [11], MUST reference 1439 only fully-qualified domain names, and MUST be effective and useful 1440 for sending replies. The translation algorithm used to convert mail 1441 from the Internet protocols to another environment's protocol SHOULD 1442 ensure that error messages from the foreign mail environment are 1443 delivered to the reverse-path from the SMTP envelope, not to an 1444 address in the "From:", "Sender:", or similar header fields of the 1445 message. 1447 3.7.5. Envelopes in Gatewaying 1449 Similarly, when forwarding a message from another environment into 1450 the Internet, the gateway SHOULD set the envelope return path in 1451 accordance with an error message return address, if supplied by the 1452 foreign environment. If the foreign environment has no equivalent 1453 concept, the gateway must select and use a best approximation, with 1454 the message originator's address as the default of last resort. 1456 3.8. Terminating Sessions and Connections 1458 An SMTP connection is terminated when the client sends a QUIT 1459 command. The server responds with a positive reply code, after which 1460 it closes the connection. 1462 An SMTP server MUST NOT intentionally close the connection under 1463 normal operational circumstances (see Section 7.8) except: 1465 o After receiving a QUIT command and responding with a 221 reply. 1467 o After detecting the need to shut down the SMTP service and 1468 returning a 421 reply code. This reply code can be issued after 1469 the server receives any command or, if necessary, asynchronously 1470 from command receipt (on the assumption that the client will 1471 receive it after the next command is issued). 1473 o After a timeout, as specified in Section 4.5.3.2, occurs waiting 1474 for the client to send a command or data. 1476 In particular, a server that closes connections in response to 1477 commands that are not understood is in violation of this 1478 specification. Servers are expected to be tolerant of unknown 1479 commands, issuing a 500 reply and awaiting further instructions from 1480 the client. 1482 An SMTP server that is forcibly shut down via external means SHOULD 1483 attempt to send a line containing a 421 reply code to the SMTP client 1484 before exiting. The SMTP client will normally read the 421 reply 1485 code after sending its next command. 1487 SMTP clients that experience a connection close, reset, or other 1488 communications failure due to circumstances not under their control 1489 (in violation of the intent of this specification but sometimes 1490 unavoidable) SHOULD, to maintain the robustness of the mail system, 1491 treat the mail transaction as if a 421 response had been received and 1492 act accordingly. 1494 3.9. Mailing Lists and Aliases 1496 [[CREF10: [5321bis] If "alias and list models" are explained 1497 elsewhere, cross reference". Also note that this section appears to 1498 prohibit an exploder from adding List-* headers. That needs to be 1499 finessed.]] 1500 An SMTP-capable host SHOULD support both the alias and the list 1501 models of address expansion for multiple delivery. When a message is 1502 delivered or forwarded to each address of an expanded list form, the 1503 return address in the envelope ("MAIL FROM:") MUST be changed to be 1504 the address of a person or other entity who administers the list. 1505 However, in this case, the message header section (RFC 5322 [11]) 1506 MUST be left unchanged; in particular, the "From" field of the header 1507 section is unaffected. 1509 An important mail facility is a mechanism for multi-destination 1510 delivery of a single message, by transforming (or "expanding" or 1511 "exploding") a pseudo-mailbox address into a list of destination 1512 mailbox addresses. When a message is sent to such a pseudo-mailbox 1513 (sometimes called an "exploder"), copies are forwarded or 1514 redistributed to each mailbox in the expanded list. Servers SHOULD 1515 simply utilize the addresses on the list; application of heuristics 1516 or other matching rules to eliminate some addresses, such as that of 1517 the originator, is strongly discouraged. We classify such a pseudo- 1518 mailbox as an "alias" or a "list", depending upon the expansion 1519 rules. 1521 3.9.1. Alias 1523 To expand an alias, the recipient mailer simply replaces the pseudo- 1524 mailbox address in the envelope with each of the expanded addresses 1525 in turn; the rest of the envelope and the message body are left 1526 unchanged. The message is then delivered or forwarded to each 1527 expanded address. 1529 3.9.2. List 1531 A mailing list may be said to operate by "redistribution" rather than 1532 by "forwarding". To expand a list, the recipient mailer replaces the 1533 pseudo-mailbox address in the envelope with each of the expanded 1534 addresses in turn. The return (backward-pointing) address in the 1535 envelope is changed so that all error messages generated by the final 1536 deliveries will be returned to a list administrator, not to the 1537 message originator, who generally has no control over the contents of 1538 the list and will typically find error messages annoying. Note that 1539 the key difference between handling aliases (Section 3.9.1) and 1540 forwarding (this subsection) is the change to the backward-pointing 1541 address in this case. When a list constrains its processing to the 1542 very limited set of modifications and actions described here, it is 1543 attempting to emulate an MTA; such lists can be treated as a 1544 continuation in email transit. 1546 There exist mailing lists that perform additional, sometimes 1547 extensive, modifications to a message and its envelope. Such mailing 1548 lists need to be viewed as full MUAs, which accept a delivery and 1549 post a new message. 1551 4. The SMTP Specifications 1553 4.1. SMTP Commands 1555 4.1.1. Command Semantics and Syntax 1557 The SMTP commands define the mail transfer or the mail system 1558 function requested by the user. SMTP commands are character strings 1559 terminated by . The commands themselves are alphabetic 1560 characters terminated by if parameters follow and 1561 otherwise. (In the interest of improved interoperability, SMTP 1562 receivers SHOULD tolerate trailing white space before the terminating 1563 .) The syntax of the local part of a mailbox MUST conform to 1564 receiver site conventions and the syntax specified in Section 4.1.2. 1565 The SMTP commands are discussed below. The SMTP replies are 1566 discussed in Section 4.2. 1568 A mail transaction involves several data objects that are 1569 communicated as arguments to different commands. The reverse-path is 1570 the argument of the MAIL command, the forward-path is the argument of 1571 the RCPT command, and the mail data is the argument of the DATA 1572 command. These arguments or data objects must be transmitted and 1573 held, pending the confirmation communicated by the end of mail data 1574 indication that finalizes the transaction. The model for this is 1575 that distinct buffers are provided to hold the types of data objects; 1576 that is, there is a reverse-path buffer, a forward-path buffer, and a 1577 mail data buffer. Specific commands cause information to be appended 1578 to a specific buffer, or cause one or more buffers to be cleared. 1580 Several commands (RSET, DATA, QUIT) are specified as not permitting 1581 parameters. In the absence of specific extensions offered by the 1582 server and accepted by the client, clients MUST NOT send such 1583 parameters and servers SHOULD reject commands containing them as 1584 having invalid syntax. 1586 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1588 These commands are used to identify the SMTP client to the SMTP 1589 server. The argument clause contains the fully-qualified domain name 1590 of the SMTP client, if one is available. In situations in which the 1591 SMTP client system does not have a meaningful domain name (e.g., when 1592 its address is dynamically allocated and no reverse mapping record is 1593 available), the client SHOULD send an address literal (see 1594 Section 4.1.3). 1596 RFC 2821, and some earlier informal practices, encouraged following 1597 the literal by information that would help to identify the client 1598 system. That convention was not widely supported, and many SMTP 1599 servers considered it an error. In the interest of interoperability, 1600 it is probably wise for servers to be prepared for this string to 1601 occur, but SMTP clients SHOULD NOT send it. 1603 The SMTP server identifies itself to the SMTP client in the 1604 connection greeting reply and in the response to this command. 1606 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1607 command. If the SMTP server supports the SMTP service extensions, it 1608 will give a successful response, a failure response, or an error 1609 response. If the SMTP server, in violation of this specification, 1610 does not support any SMTP service extensions, it will generate an 1611 error response. Older client SMTP systems MAY, as discussed above, 1612 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1613 support the HELO command and reply properly to it. In any event, a 1614 client MUST issue HELO or EHLO before starting a mail transaction. 1616 These commands, and a "250 OK" reply to one of them, confirm that 1617 both the SMTP client and the SMTP server are in the initial state, 1618 that is, there is no transaction in progress and all state tables and 1619 buffers are cleared. 1621 Syntax: 1623 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1625 helo = "HELO" SP Domain CRLF 1627 Normally, the response to EHLO will be a multiline reply. Each line 1628 of the response contains a keyword and, optionally, one or more 1629 parameters. Following the normal syntax for multiline replies, these 1630 keywords follow the code (250) and a hyphen for all but the last 1631 line, and the code and a space for the last line. The syntax for a 1632 positive response, using the ABNF notation and terminal symbols of 1633 RFC 5234 [10], is: 1635 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1636 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1637 *( "250-" ehlo-line CRLF ) 1638 "250" SP ehlo-line CRLF ) 1640 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1641 ; string of any characters other than CR or LF 1643 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1645 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1646 ; additional syntax of ehlo-params depends on 1647 ; ehlo-keyword 1649 ehlo-param = 1*(%d33-126) 1650 ; any CHAR excluding and all 1651 ; control characters (US-ASCII 0-31 and 127 1652 ; inclusive) 1654 Although EHLO keywords may be specified in upper, lower, or mixed 1655 case, they MUST always be recognized and processed in a case- 1656 insensitive manner. This is simply an extension of practices 1657 specified in RFC 821 and Section 2.4. 1659 The EHLO response MUST contain keywords (and associated parameters if 1660 required) for all commands not listed as "required" in Section 4.5.1 1661 excepting only private-use commands as described in Section 4.1.5. 1662 Private-use commands MAY be listed. 1664 4.1.1.2. MAIL (MAIL) 1666 This command is used to initiate a mail transaction in which the mail 1667 data is delivered to an SMTP server that may, in turn, deliver it to 1668 one or more mailboxes or pass it on to another system (possibly using 1669 SMTP). The argument clause contains a reverse-path and may contain 1670 optional parameters. In general, the MAIL command may be sent only 1671 when no mail transaction is in progress, see Section 4.1.4. 1673 The reverse-path consists of the sender mailbox. Historically, that 1674 mailbox might optionally have been preceded by a list of hosts, but 1675 that behavior is now deprecated (see Appendix C). In some types of 1676 reporting messages for which a reply is likely to cause a mail loop 1677 (for example, mail delivery and non-delivery notifications), the 1678 reverse-path may be null (see Section 3.6). 1680 This command clears the reverse-path buffer, the forward-path buffer, 1681 and the mail data buffer, and it inserts the reverse-path information 1682 from its argument clause into the reverse-path buffer. 1684 If service extensions were negotiated, the MAIL command may also 1685 carry parameters associated with a particular service extension. 1687 Syntax: 1689 mail = "MAIL FROM:" Reverse-path 1690 [SP Mail-parameters] CRLF 1692 4.1.1.3. RECIPIENT (RCPT) 1694 This command is used to identify an individual recipient of the mail 1695 data; multiple recipients are specified by multiple uses of this 1696 command. The argument clause contains a forward-path and may contain 1697 optional parameters. 1699 The forward-path normally consists of the required destination 1700 mailbox. Sending systems SHOULD NOT generate the optional list of 1701 hosts known as a source route. Receiving systems MUST recognize 1702 source route syntax but SHOULD strip off the source route 1703 specification and utilize the domain name associated with the mailbox 1704 as if the source route had not been provided. 1706 Similarly, relay hosts SHOULD strip or ignore source routes, and 1707 names MUST NOT be copied into the reverse-path. When mail reaches 1708 its ultimate destination (the forward-path contains only a 1709 destination mailbox), the SMTP server inserts it into the destination 1710 mailbox in accordance with its host mail conventions. 1712 This command appends its forward-path argument to the forward-path 1713 buffer; it does not change the reverse-path buffer nor the mail data 1714 buffer. 1716 For example, mail received at relay host xyz.com with envelope 1717 commands 1719 MAIL FROM: 1720 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1722 will normally be sent directly on to host d.bar.org with envelope 1723 commands 1725 MAIL FROM: 1726 RCPT TO: 1728 As provided in Appendix C, xyz.com MAY also choose to relay the 1729 message to hosta.int, using the envelope commands 1731 MAIL FROM: 1732 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1734 or to jkl.org, using the envelope commands 1736 MAIL FROM: 1737 RCPT TO:<@jkl.org:userc@d.bar.org> 1739 Attempting to use relaying this way is now strongly discouraged. 1740 Since hosts are not required to relay mail at all, xyz.com MAY also 1741 reject the message entirely when the RCPT command is received, using 1742 a 550 code (since this is a "policy reason"). 1744 If service extensions were negotiated, the RCPT command may also 1745 carry parameters associated with a particular service extension 1746 offered by the server. The client MUST NOT transmit parameters other 1747 than those associated with a service extension offered by the server 1748 in its EHLO response. 1750 Syntax: 1752 rcpt = "RCPT TO:" ( "" / "" / 1753 Forward-path ) [SP Rcpt-parameters] CRLF 1755 Note that, in a departure from the usual rules for 1756 local-parts, the "Postmaster" string shown above is 1757 treated as case-insensitive. 1759 4.1.1.4. DATA (DATA) 1761 The receiver normally sends a 354 response to DATA, and then treats 1762 the lines (strings ending in sequences, as described in 1763 Section 2.3.7) following the command as mail data from the sender. 1764 This command causes the mail data to be appended to the mail data 1765 buffer. The mail data may contain any of the 128 ASCII character 1766 codes, although experience has indicated that use of control 1767 characters other than SP, HT, CR, and LF may cause problems and 1768 SHOULD be avoided when possible. 1770 The mail data are terminated by a line containing only a period, that 1771 is, the character sequence ".", where the first is 1772 actually the terminator of the previous line (see Section 4.5.2). 1773 This is the end of mail data indication. The first of this 1774 terminating sequence is also the that ends the final line of 1775 the data (message text) or, if there was no mail data, ends the DATA 1776 command itself (the "no mail data" case does not conform to this 1777 specification since it would require that neither the trace header 1778 fields required by this specification nor the message header section 1779 required by RFC 5322 [11] be transmitted). An extra MUST NOT 1780 be added, as that would cause an empty line to be added to the 1781 message. The only exception to this rule would arise if the message 1782 body were passed to the originating SMTP-sender with a final "line" 1783 that did not end in ; in that case, the originating SMTP system 1784 MUST either reject the message as invalid or add in order to 1785 have the receiving SMTP server recognize the "end of data" condition. 1787 The custom of accepting lines ending only in , as a concession to 1788 non-conforming behavior on the part of some UNIX systems, has proven 1789 to cause more interoperability problems than it solves, and SMTP 1790 server systems MUST NOT do this, even in the name of improved 1791 robustness. In particular, the sequence "." (bare line 1792 feeds, without carriage returns) MUST NOT be treated as equivalent to 1793 . as the end of mail data indication. 1795 Receipt of the end of mail data indication requires the server to 1796 process the stored mail transaction information. This processing 1797 consumes the information in the reverse-path buffer, the forward-path 1798 buffer, and the mail data buffer, and on the completion of this 1799 command these buffers are cleared. If the processing is successful, 1800 the receiver MUST send an OK reply. If the processing fails, the 1801 receiver MUST send a failure reply. The SMTP model does not allow 1802 for partial failures at this point: either the message is accepted by 1803 the server for delivery and a positive response is returned or it is 1804 not accepted and a failure reply is returned. In sending a positive 1805 "250 OK" completion reply to the end of data indication, the receiver 1806 takes full responsibility for the message (see Section 6.1). Errors 1807 that are diagnosed subsequently MUST be reported in a mail message, 1808 as discussed in Section 4.4. 1810 When the SMTP server accepts a message either for relaying or for 1811 final delivery, it inserts a trace record (also referred to 1812 interchangeably as a "time stamp line" or "Received" line) at the top 1813 of the mail data. This trace record indicates the identity of the 1814 host that sent the message, the identity of the host that received 1815 the message (and is inserting this time stamp), and the date and time 1816 the message was received. Relayed messages will have multiple time 1817 stamp lines. Details for formation of these lines, including their 1818 syntax, is specified in Section 4.4. 1820 Additional discussion about the operation of the DATA command appears 1821 in Section 3.3. 1823 Syntax: 1825 data = "DATA" CRLF 1827 4.1.1.5. RESET (RSET) 1829 This command specifies that the current mail transaction will be 1830 aborted. Any stored sender, recipients, and mail data MUST be 1831 discarded, and all buffers and state tables cleared. The receiver 1832 MUST send a "250 OK" reply to a RSET command with no arguments. A 1833 reset command may be issued by the client at any time. It is 1834 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1835 immediately after EHLO, before EHLO is issued in the session, after 1836 an end of data indicator has been sent and acknowledged, or 1837 immediately before a QUIT. An SMTP server MUST NOT close the 1838 connection as the result of receiving a RSET; that action is reserved 1839 for QUIT (see Section 4.1.1.10). 1841 Since EHLO implies some additional processing and response by the 1842 server, RSET will normally be more efficient than reissuing that 1843 command, even though the formal semantics are the same. 1845 There are circumstances, contrary to the intent of this 1846 specification, in which an SMTP server may receive an indication that 1847 the underlying TCP connection has been closed or reset. To preserve 1848 the robustness of the mail system, SMTP servers SHOULD be prepared 1849 for this condition and SHOULD treat it as if a QUIT had been received 1850 before the connection disappeared. 1852 Syntax: 1854 rset = "RSET" CRLF 1856 4.1.1.6. VERIFY (VRFY) 1858 This command asks the receiver to confirm that the argument 1859 identifies a user or mailbox. If it is a user name, information is 1860 returned as specified in Section 3.5. 1862 This command has no effect on the reverse-path buffer, the forward- 1863 path buffer, or the mail data buffer. 1865 Syntax: 1867 vrfy = "VRFY" SP String CRLF 1869 4.1.1.7. EXPAND (EXPN) 1871 This command asks the receiver to confirm that the argument 1872 identifies a mailing list, and if so, to return the membership of 1873 that list. If the command is successful, a reply is returned 1874 containing information as described in Section 3.5. This reply will 1875 have multiple lines except in the trivial case of a one-member list. 1877 This command has no effect on the reverse-path buffer, the forward- 1878 path buffer, or the mail data buffer, and it may be issued at any 1879 time. 1881 Syntax: 1883 expn = "EXPN" SP String CRLF 1885 4.1.1.8. HELP (HELP) 1887 This command causes the server to send helpful information to the 1888 client. The command MAY take an argument (e.g., any command name) 1889 and return more specific information as a response. 1891 This command has no effect on the reverse-path buffer, the forward- 1892 path buffer, or the mail data buffer, and it may be issued at any 1893 time. 1895 SMTP servers SHOULD support HELP without arguments and MAY support it 1896 with arguments. 1898 Syntax: 1900 help = "HELP" [ SP String ] CRLF 1902 4.1.1.9. NOOP (NOOP) 1904 This command does not affect any parameters or previously entered 1905 commands. It specifies no action other than that the receiver send a 1906 "250 OK" reply. 1908 This command has no effect on the reverse-path buffer, the forward- 1909 path buffer, or the mail data buffer, and it may be issued at any 1910 time. If a parameter string is specified, servers SHOULD ignore it. 1912 Syntax: 1914 noop = "NOOP" [ SP String ] CRLF 1916 4.1.1.10. QUIT (QUIT) 1918 This command specifies that the receiver MUST send a "221 OK" reply, 1919 and then close the transmission channel. 1921 The receiver MUST NOT intentionally close the transmission channel 1922 until it receives and replies to a QUIT command (even if there was an 1923 error). The sender MUST NOT intentionally close the transmission 1924 channel until it sends a QUIT command, and it SHOULD wait until it 1925 receives the reply (even if there was an error response to a previous 1926 command). If the connection is closed prematurely due to violations 1927 of the above or system or network failure, the server MUST cancel any 1928 pending transaction, but not undo any previously completed 1929 transaction, and generally MUST act as if the command or transaction 1930 in progress had received a temporary error (i.e., a 4yz response). 1932 The QUIT command may be issued at any time. Any current uncompleted 1933 mail transaction will be aborted. 1935 Syntax: 1937 quit = "QUIT" CRLF 1939 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1941 If the server SMTP does not recognize or cannot implement one or more 1942 of the parameters associated with a particular MAIL FROM or RCPT TO 1943 command, it will return code 555. 1945 If, for some reason, the server is temporarily unable to accommodate 1946 one or more of the parameters associated with a MAIL FROM or RCPT TO 1947 command, and if the definition of the specific parameter does not 1948 mandate the use of another code, it should return code 455. 1950 Errors specific to particular parameters and their values will be 1951 specified in the parameter's defining RFC. 1953 4.1.2. Command Argument Syntax 1955 The syntax of the argument clauses of the above commands (using the 1956 syntax specified in RFC 5234 [10] where applicable) is given below. 1957 Some of the productions given below are used only in conjunction with 1958 source routes as described in Appendix C. Some terminals not defined 1959 in this document, but are defined elsewhere, specifically: 1961 In the "core" syntax in Appendix B of RFC 5234 [10]: ALPHA , CRLF 1962 , DIGIT , HEXDIG , and SP 1964 In the message format syntax in RFC 5322 [11]: atext , CFWS , and 1965 FWS . 1967 Reverse-path = Path / "<>" 1969 Forward-path = Path 1971 Path = "<" [ A-d-l ":" ] Mailbox ">" 1973 A-d-l = At-domain *( "," At-domain ) 1974 ; Note that this form, the so-called "source 1975 ; route", MUST BE accepted, SHOULD NOT be 1976 ; generated, and SHOULD be ignored. 1978 At-domain = "@" Domain 1980 Mail-parameters = esmtp-param *(SP esmtp-param) 1982 Rcpt-parameters = esmtp-param *(SP esmtp-param) 1984 esmtp-param = esmtp-keyword ["=" esmtp-value] 1986 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1988 esmtp-value = 1*(%d33-60 / %d62-126) 1989 ; any CHAR excluding "=", SP, and control 1990 ; characters. If this string is an email address, 1991 ; i.e., a Mailbox, then the "xtext" syntax [33] 1992 ; SHOULD be used. 1994 Keyword = Ldh-str 1996 Argument = Atom 1997 Domain = sub-domain *("." sub-domain) 1999 sub-domain = Let-dig [Ldh-str] 2001 Let-dig = ALPHA / DIGIT 2003 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2005 address-literal = "[" ( IPv4-address-literal / 2006 IPv6-address-literal / 2007 General-address-literal ) "]" 2008 ; See Section 4.1.3 2010 Mailbox = Local-part "@" ( Domain / address-literal ) 2012 Local-part = Dot-string / Quoted-string 2013 ; MAY be case-sensitive 2015 Dot-string = Atom *("." Atom) 2017 Atom = 1*atext 2019 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2021 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2023 quoted-pairSMTP = %d92 %d32-126 2024 ; i.e., backslash followed by any ASCII 2025 ; graphic (including itself) or SPace 2027 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2028 ; i.e., within a quoted string, any 2029 ; ASCII graphic or space is permitted 2030 ; without blackslash-quoting except 2031 ; double-quote and the backslash itself. 2033 String = Atom / Quoted-string 2035 While the above definition for Local-part is relatively permissive, 2036 for maximum interoperability, a host that expects to receive mail 2037 SHOULD avoid defining mailboxes where the Local-part requires (or 2038 uses) the Quoted-string form or where the Local-part is case- 2039 sensitive. For any purposes that require generating or comparing 2040 Local-parts (e.g., to specific mailbox names), all quoted forms MUST 2041 be treated as equivalent, and the sending system SHOULD transmit the 2042 form that uses the minimum quoting possible. 2044 Systems MUST NOT define mailboxes in such a way as to require the use 2045 in SMTP of non-ASCII characters (octets with the high order bit set 2046 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2047 These characters MUST NOT be used in MAIL or RCPT commands or other 2048 commands that require mailbox names. 2050 Note that the backslash, "\", is a quote character, which is used to 2051 indicate that the next character is to be used literally (instead of 2052 its normal interpretation). For example, "Joe\,Smith" indicates a 2053 single nine-character user name string with the comma being the 2054 fourth character of that string. 2056 To promote interoperability and consistent with long-standing 2057 guidance about conservative use of the DNS in naming and applications 2058 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2059 characters outside the set of alphabetic characters, digits, and 2060 hyphen MUST NOT appear in domain name labels for SMTP clients or 2061 servers. In particular, the underscore character is not permitted. 2062 SMTP servers that receive a command in which invalid character codes 2063 have been employed, and for which there are no other reasons for 2064 rejection, MUST reject that command with a 501 response (this rule, 2065 like others, could be overridden by appropriate SMTP extensions). 2067 4.1.3. Address Literals 2069 Sometimes a host is not known to the domain name system and 2070 communication (and, in particular, communication to report and repair 2071 the error) is blocked. To bypass this barrier, a special literal 2072 form of the address is allowed as an alternative to a domain name. 2073 For IPv4 addresses, this form uses four small decimal integers 2074 separated by dots and enclosed by brackets such as [123.255.37.2], 2075 which indicates an (IPv4) Internet Address in sequence-of-octets 2076 form. For IPv6 and other forms of addressing that might eventually 2077 be standardized, the form consists of a standardized "tag" that 2078 identifies the address syntax, a colon, and the address itself, in a 2079 format specified as part of the relevant standards (i.e., RFC 4291 2080 [9] for IPv6). 2081 [[CREF11: [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet 2082 another modification to the IPv6 address literal syntax, based on 2083 part on RFC 5952. We should consider whether those, or other, 2084 modifications are appropriate and/or whether, given both the issues 2085 of spam/malware and servers supporting multiple domains, it it time 2086 to deprecate mailboxes containing address literals entirely (EHLO 2087 fields may be a different issue). If we are going to allow IPv6 2088 address literals, it may be time to incorporate something by 2089 reference rather than including specific syntax here (RFC 5952 is 14 2090 pages long and does not contain any ABNF).]] 2091 Specifically: 2093 IPv4-address-literal = Snum 3("." Snum) 2095 IPv6-address-literal = "IPv6:" IPv6-addr 2097 General-address-literal = Standardized-tag ":" 1*dcontent 2099 Standardized-tag = Ldh-str 2100 ; Standardized-tag MUST be specified in a 2101 ; Standards-Track RFC and registered with IANA 2103 dcontent = %d33-90 / ; Printable US-ASCII 2104 %d94-126 ; excl. "[", "\", "]" 2106 Snum = 1*3DIGIT 2107 ; representing a decimal integer 2108 ; value in the range 0 through 255 2110 IPv6-addr = 6( h16 ":" ) ls32 2111 / "::" 5( h16 ":" ) ls32 2112 / [ h16 ] "::" 4( h16 ":" ) ls32 2113 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2114 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2115 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2116 / [ *4( h16 ":" ) h16 ] "::" ls32 2117 / [ *5( h16 ":" ) h16 ] "::" h16 2118 / [ *6( h16 ":" ) h16 ] "::" 2119 ; This definition is consistent with the one for 2120 ; URIs [40]. 2122 ls32 = ( h16 ":" h16 ) / IPv4address 2123 ; least-significant 32 bits of address 2125 h16 = 1*4HEXDIG 2126 ; 16 bits of address represented in hexadecimal 2127 [[CREF12: [5321bis](2821ter) 2821bis Last Call 2128 comment]] 2130 4.1.4. Order of Commands 2132 There are restrictions on the order in which these commands may be 2133 used. 2135 A session that will contain mail transactions MUST first be 2136 initialized by the use of the EHLO command. An SMTP server SHOULD 2137 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2138 without this initialization. 2140 An EHLO command MAY be issued by a client later in the session. If 2141 it is issued after the session begins and the EHLO command is 2142 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2143 and reset the state exactly as if a RSET command had been issued. In 2144 other words, the sequence of RSET followed immediately by EHLO is 2145 redundant, but not harmful other than in the performance cost of 2146 executing unnecessary commands. 2148 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2149 502, or 550 failure replies MUST be returned as appropriate. The 2150 SMTP server MUST stay in the same state after transmitting these 2151 replies that it was in before the EHLO was received. 2153 The SMTP client MUST, if possible, ensure that the domain parameter 2154 to the EHLO command is a primary host name as specified for this 2155 command in Section 2.3.5. If this is not possible (e.g., when the 2156 client's address is dynamically assigned and the client does not have 2157 an obvious name), an address literal SHOULD be substituted for the 2158 domain name. 2160 An SMTP server MAY verify that the domain name argument in the EHLO 2161 command actually corresponds to the IP address of the client. 2162 [[CREF13: [5321bis] [[Note in draft -- proposed change to "An SMTP 2163 server MAY verify that the domain name argument in the EHLO command 2164 has an address record matching the IP address of the client." --David 2165 MacQuigg, david_macquigg@yahoo.com, Friday, 20090130 0637 -0700]]]] 2166 However, if the verification fails, the server MUST NOT refuse to 2167 accept a message on that basis. Information captured in the 2168 verification attempt is for logging and tracing purposes. Note that 2169 this prohibition applies to the matching of the parameter to its IP 2170 address only; see Section 7.9 for a more extensive discussion of 2171 rejecting incoming connections or mail messages. 2173 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2174 during a session, or without previously initializing a session. SMTP 2175 servers SHOULD process these normally (that is, not return a 503 2176 code) even if no EHLO command has yet been received; clients SHOULD 2177 open a session with EHLO before sending these commands. 2179 If these rules are followed, the example in RFC 821 that shows "550 2180 access denied to you" in response to an EXPN command is incorrect 2181 unless an EHLO command precedes the EXPN or the denial of access is 2182 based on the client's IP address or other authentication or 2183 authorization-determining mechanisms. 2185 The MAIL command (or the obsolete SEND, SOML, or SAML commands) 2187 [[5321bis Editor's Note: is there any reason to not clean those 2188 commands out of this entirely, replacing them with, e.g., a strong 2189 reference to Appendix F.6]] 2190 begins a mail transaction. Once started, a mail transaction consists 2191 of a transaction beginning command, one or more RCPT commands, and a 2192 DATA command, in that order. A mail transaction may be aborted by 2193 the RSET, a new EHLO, or the QUIT command. There may be zero or more 2194 transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be 2195 sent if a mail transaction is already open, i.e., it should be sent 2196 only if no mail transaction had been started in the session, or if 2197 the previous one successfully concluded with a successful DATA 2198 command, or if the previous one was aborted, e.g., with a RSET or new 2199 EHLO. [[CREF14: [5321bis] 2821ter note: see comment about changing 2200 this convoluted discussion to talk about 'mail transaction' above. 2201 --Jck]] 2203 If the transaction beginning command argument is not acceptable, a 2204 501 failure reply MUST be returned and the SMTP server MUST stay in 2205 the same state. If the commands in a transaction are out of order to 2206 the degree that they cannot be processed by the server, a 503 failure 2207 reply MUST be returned and the SMTP server MUST stay in the same 2208 state. 2210 The last command in a session MUST be the QUIT command. The QUIT 2211 command SHOULD be used by the client SMTP to request connection 2212 closure, even when no session opening command was sent and accepted. 2214 4.1.5. Private-Use Commands 2216 As specified in Section 2.2.2, commands starting in "X" may be used 2217 by bilateral agreement between the client (sending) and server 2218 (receiving) SMTP agents. An SMTP server that does not recognize such 2219 a command is expected to reply with "500 Command not recognized". An 2220 extended SMTP server MAY list the feature names associated with these 2221 private commands in the response to the EHLO command. 2223 Commands sent or accepted by SMTP systems that do not start with "X" 2224 MUST conform to the requirements of Section 2.2.2. 2226 4.2. SMTP Replies 2228 Replies to SMTP commands serve to ensure the synchronization of 2229 requests and actions in the process of mail transfer and to guarantee 2230 that the SMTP client always knows the state of the SMTP server. 2231 Every command MUST generate exactly one reply. 2233 The details of the command-reply sequence are described in 2234 Section 4.3. 2236 An SMTP reply consists of a three digit number (transmitted as three 2237 numeric characters) followed by some text unless specified otherwise 2238 in this document. The number is for use by automata to determine 2239 what state to enter next; the text is for the human user. The three 2240 digits contain enough encoded information that the SMTP client need 2241 not examine the text and may either discard it or pass it on to the 2242 user, as appropriate. Exceptions are as noted elsewhere in this 2243 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2244 are associated with message text that must be parsed and interpreted 2245 by machines. In the general case, the text may be receiver dependent 2246 and context dependent, so there are likely to be varying texts for 2247 each reply code. A discussion of the theory of reply codes is given 2248 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2249 three-digit code, , one line of text, and , or a multiline 2250 reply (as defined in the same section). Since, in violation of this 2251 specification, the text is sometimes not sent, clients that do not 2252 receive it SHOULD be prepared to process the code alone (with or 2253 without a trailing space character). Only the EHLO, EXPN, and HELP 2254 commands are expected to result in multiline replies in normal 2255 circumstances; however, multiline replies are allowed for any 2256 command. 2258 In ABNF, server responses are: 2260 Greeting = ( "220 " (Domain / address-literal) 2261 [ SP textstring ] CRLF ) / 2262 ( "220-" (Domain / address-literal) 2263 [ SP textstring ] CRLF 2264 *( "220-" [ textstring ] CRLF ) 2265 "220" [ SP textstring ] CRLF ) 2267 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2269 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2270 Reply-code [ SP textstring ] CRLF 2272 Reply-code = %x32-35 %x30-35 %x30-39 2274 where "Greeting" appears only in the 220 response that announces that 2275 the server is opening its part of the connection. (Other possible 2276 server responses upon connection follow the syntax of Reply-line.) 2278 An SMTP server SHOULD send only the reply codes listed in this 2279 document or additions to the list as discussed below. 2280 [[CREF15: [5321bis] 20140804: New text to clear up ambiguity.]] 2281 An SMTP server SHOULD use the text shown in the examples whenever 2282 appropriate. 2284 An SMTP client MUST determine its actions only by the reply code, not 2285 by the text (except for the "change of address" 251 and 551 and, if 2286 necessary, 220, 221, and 421 replies); in the general case, any text, 2287 including no text at all (although senders SHOULD NOT send bare 2288 codes), MUST be acceptable. The space (blank) following the reply 2289 code is considered part of the text. Whenever possible, a sender- 2290 SMTP SHOULD test the first digit (severity indication) of a reply 2291 code it receives. 2292 [[CREF16: [5321bis] 20141209 [[Note in Draft: What is that sentence 2293 supposed to be tell us? Test the first digit and examine the others 2294 only if necessary? Note the interaction between this and various 2295 flaps about adding new codes.]]]] 2297 The list of codes that appears below MUST NOT be construed as 2298 permanent. While the addition of new codes should be a rare and 2299 significant activity, with supplemental information in the textual 2300 part of the response (including enhanced status codes [34] and the 2301 successors to that specification) 2302 [[CREF17: [5321bis] 20140802: New text for clarity]] 2303 being preferred, new codes may be added as the result of new 2304 Standards or Standards-Track specifications. Consequently, a sender- 2305 SMTP MUST be prepared to handle codes not specified in this document 2306 and MUST do so by interpreting the first digit only. 2308 In the absence of extensions negotiated with the client, SMTP servers 2309 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2310 or 5. Clients that receive such out-of-range codes SHOULD normally 2311 treat them as fatal errors and terminate the mail transaction. 2313 4.2.1. Reply Code Severities and Theory 2315 The three digits of the reply each have a special significance. The 2316 first digit denotes whether the response is good, bad, or incomplete. 2317 An unsophisticated SMTP client, or one that receives an unexpected 2318 code, will be able to determine its next action (proceed as planned, 2319 redo, retrench, etc.) by examining this first digit. An SMTP client 2320 that wants to know approximately what kind of error occurred (e.g., 2321 mail system error, command syntax error) may examine the second 2322 digit. The third digit and any supplemental information that may be 2323 present is reserved for the finest gradation of information. 2325 There are four values for the first digit of the reply code: 2327 2yz Positive Completion reply 2328 The requested action has been successfully completed. A new 2329 request may be initiated. 2331 3yz Positive Intermediate reply 2332 The command has been accepted, but the requested action is being 2333 held in abeyance, pending receipt of further information. The 2334 SMTP client should send another command specifying this 2335 information. This reply is used in command sequence groups (i.e., 2336 in DATA). 2338 4yz Transient Negative Completion reply 2339 The command was not accepted, and the requested action did not 2340 occur. However, the error condition is temporary, and the action 2341 may be requested again. The sender should return to the beginning 2342 of the command sequence (if any). It is difficult to assign a 2343 meaning to "transient" when two different sites (receiver- and 2344 sender-SMTP agents) must agree on the interpretation. Each reply 2345 in this category might have a different time value, but the SMTP 2346 client SHOULD try again. A rule of thumb to determine whether a 2347 reply fits into the 4yz or the 5yz category (see below) is that 2348 replies are 4yz if they can be successful if repeated without any 2349 change in command form or in properties of the sender or receiver 2350 (that is, the command is repeated identically and the receiver 2351 does not put up a new implementation). 2353 5yz Permanent Negative Completion reply 2354 The command was not accepted and the requested action did not 2355 occur. The SMTP client SHOULD NOT repeat the exact request (in 2356 the same sequence). Even some "permanent" error conditions can be 2357 corrected, so the human user may want to direct the SMTP client to 2358 reinitiate the command sequence by direct action at some point in 2359 the future (e.g., after the spelling has been changed, or the user 2360 has altered the account status). 2362 It is worth noting that the file transfer protocol (FTP) [14] uses a 2363 very similar code architecture and that the SMTP codes are based on 2364 the FTP model. However, SMTP uses a one-command, one-response model 2365 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2366 SMTP model. 2368 The second digit encodes responses in specific categories: 2370 x0z Syntax: These replies refer to syntax errors, syntactically 2371 correct commands that do not fit any functional category, and 2372 unimplemented or superfluous commands. 2374 x1z Information: These are replies to requests for information, such 2375 as status or help. 2377 x2z Connections: These are replies referring to the transmission 2378 channel. 2380 x3z Unspecified. 2382 x4z Unspecified. 2384 x5z Mail system: These replies indicate the status of the receiver 2385 mail system vis-a-vis the requested transfer or other mail system 2386 action. 2388 The third digit gives a finer gradation of meaning in each category 2389 specified by the second digit. The list of replies illustrates this. 2390 Each reply text is recommended rather than mandatory, and may even 2391 change according to the command with which it is associated. On the 2392 other hand, the reply codes must strictly follow the specifications 2393 in this section. Receiver implementations should not invent new 2394 codes for slightly different situations from the ones described here, 2395 but rather adapt codes already defined. 2397 For example, a command such as NOOP, whose successful execution does 2398 not offer the SMTP client any new information, will return a 250 2399 reply. The reply is 502 when the command requests an unimplemented 2400 non-site-specific action. A refinement of that is the 504 reply for 2401 a command that is implemented, but that requests an unimplemented 2402 parameter. 2404 The reply text may be longer than a single line; in these cases the 2405 complete text must be marked so the SMTP client knows when it can 2406 stop reading the reply. This requires a special format to indicate a 2407 multiple line reply. 2409 The format for multiline replies requires that every line, except the 2410 last, begin with the reply code, followed immediately by a hyphen, 2411 "-" (also known as minus), followed by text. The last line will 2412 begin with the reply code, followed immediately by , optionally 2413 some text, and . As noted above, servers SHOULD send the 2414 if subsequent text is not sent, but clients MUST be prepared for it 2415 to be omitted. 2417 For example: 2419 250-First line 2420 250-Second line 2421 250-234 Text beginning with numbers 2422 250 The last line 2424 In a multiline reply, the reply code on each of the lines MUST be the 2425 same. It is reasonable for the client to rely on this, so it can 2426 make processing decisions based on the code in any line, assuming 2427 that all others will be the same. In a few cases, there is important 2428 data for the client in the reply "text". The client will be able to 2429 identify these cases from the current context. 2431 4.2.2. Reply Codes by Function Groups 2433 500 Syntax error, command unrecognized (This may include errors such 2434 as command line too long) 2436 501 Syntax error in parameters or arguments 2438 502 Command not implemented (see Section 4.2.4.1) 2440 503 Bad sequence of commands 2442 504 Command parameter not implemented 2444 211 System status, or system help reply 2446 214 Help message (Information on how to use the receiver or the 2447 meaning of a particular non-standard command; this reply is useful 2448 only to the human user) 2450 220 Service ready 2452 221 Service closing transmission channel 2454 421 Service not available, closing transmission channel 2455 (This may be a reply to any command if the service knows it must 2456 shut down) 2458 hangText="521"> No mail service here. [[CREF18: 2459 [5321bis]20140804: Specific code introduced with RFC 1846, updated 2460 and specified in draft-klensin-smtp-521code.]] 2462 556 No mail service at this domain. [[CREF19: [5321bis] 20140912: 2463 Specific code introduced in draft-klensin-smtp-521code-02 (RFC 2464 7504), largely for nullMX]] 2466 250 Requested mail action okay, completed 2468 251 User not local; will forward to (See Section 3.4) 2470 252 Cannot VRFY user, but will accept message and attempt delivery 2471 (See Section 3.5.3) 2473 455 Server unable to accommodate parameters 2475 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2477 450 Requested mail action not taken: mailbox unavailable (e.g., 2478 mailbox busy or temporarily blocked for policy reasons) 2480 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2481 not found, no access, or command rejected for policy reasons) 2483 451 Requested action aborted: error in processing 2485 551 User not local; please try (See Section 3.4) 2487 452 Requested action not taken: insufficient system storage 2489 552 Requested mail action aborted: exceeded storage allocation 2491 553 Requested action not taken: mailbox name not allowed (e.g., 2492 mailbox syntax incorrect) 2494 354 Start mail input; end with . 2496 554 Transaction failed (Or, in the case of a connection-opening 2497 response, "No SMTP service here") 2498 [[CREF20: [5321bis] [[Note in Draft: Revise above statement in the 2499 light of new 521 code??]] ]] 2501 4.2.3. Reply Codes in Numeric Order 2503 211 System status, or system help reply 2505 214 Help message (Information on how to use the receiver or the 2506 meaning of a particular non-standard command; this reply is useful 2507 only to the human user) 2509 220 Service ready 2511 221 Service closing transmission channel 2513 250 Requested mail action okay, completed 2515 251 User not local; will forward to (See Section 3.4) 2517 252 Cannot VRFY user, but will accept message and attempt delivery 2518 (See Section 3.5.3) 2520 354 Start mail input; end with . 2522 421 Service not available, closing transmission channel 2523 (This may be a reply to any command if the service knows it must 2524 shut down) 2526 450 Requested mail action not taken: mailbox unavailable (e.g., 2527 mailbox busy or temporarily blocked for policy reasons) 2529 451 Requested action aborted: local error in processing 2531 452 Requested action not taken: insufficient system storage 2533 455 Server unable to accommodate parameters 2535 500 Syntax error, command unrecognized (This may include errors such 2536 as command line too long) 2538 501 Syntax error in parameters or arguments 2540 502 Command not implemented (see Section 4.2.4.1) 2542 503 Bad sequence of commands 2544 504 Command parameter not implemented 2546 521 No mail service 2548 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2549 not found, no access, or command rejected for policy reasons) 2551 551 User not local; please try (See Section 3.4) 2553 552 Requested mail action aborted: exceeded storage allocation 2555 553 Requested action not taken: mailbox name not allowed (e.g., 2556 mailbox syntax incorrect) 2558 554 Transaction failed (Or, in the case of a connection-opening 2559 response, "No SMTP service here") 2561 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2563 556 No mail service at this domain. 2565 4.2.4. Some specific code situations and relationships 2567 4.2.4.1. Reply Code 502 2569 Questions have been raised as to when reply code 502 (Command not 2570 implemented) SHOULD be returned in preference to other codes. 502 2571 SHOULD be used when the command is actually recognized by the SMTP 2572 server, but not implemented. If the command is not recognized, code 2573 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2574 capabilities in response to EHLO for which they will return 502 (or 2575 500) replies. 2577 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2579 [[CREF21: [5321bis] This section is new with 5321bis. ]] 2581 Codes 521, 554, and 556 are all used to report different types of "no 2582 mail accepted" situations. They differ as follows. 521 is an 2583 indication from a system answering on the SMTP port that it does not 2584 support SMTP service (a so-called "dummy server" as discussed in RFC 2585 1846 [19] and elsewhere). Obviously, it requires that system exist 2586 and that a connection can be made successfully to it. Because a 2587 system that does not accept any mail cannot meaningfully accept a 2588 RCPT command, any commands (other than QUIT) issued after an SMTP 2589 server has issued a 521 reply are client (sender) errors. 556 is 2590 used by a message submission or intermediate SMTP system (see 2591 Section 1.1) to report that it cannot forward the message further 2592 because it knows (e.g., from a DNS entry [51]) that the recipient 2593 domain does not accept mail. It would normally be used in response 2594 to a RCPT or similar (extension) command when the SMTP system 2595 identifies a domain that it can (or has) determined never accepts 2596 mail. Other codes, including 554 and the temporary 450, are used for 2597 more transient situations and situations in which an SMTP server 2598 cannot or will not deliver to (or accept mail for) a particular 2599 system or mailbox for policy reasons rather than ones directly 2600 related to SMTP processing. 2602 4.2.4.3. Reply Codes after DATA and the Subsequent . 2604 When an SMTP server returns a positive completion status (2yz code) 2605 after the DATA command is completed with ., it accepts 2606 responsibility for: 2608 o delivering the message (if the recipient mailbox exists), or 2610 o if attempts to deliver the message fail due to transient 2611 conditions, retrying delivery some reasonable number of times at 2612 intervals as specified in Section 4.5.4. 2614 o if attempts to deliver the message fail due to permanent 2615 conditions, or if repeated attempts to deliver the message fail 2616 due to transient conditions, returning appropriate notification to 2617 the sender of the original message (using the address in the SMTP 2618 MAIL command). 2620 When an SMTP server returns a temporary error status (4yz) code after 2621 the DATA command is completed with ., it MUST NOT make a 2622 subsequent attempt to deliver that message. The SMTP client retains 2623 responsibility for the delivery of that message and may either return 2624 it to the user or requeue it for a subsequent attempt (see 2625 Section 4.5.4.1). 2627 The user who originated the message SHOULD be able to interpret the 2628 return of a transient failure status (by mail message or otherwise) 2629 as a non-delivery indication, just as a permanent failure would be 2630 interpreted. If the client SMTP successfully handles these 2631 conditions, the user will not receive such a reply. 2633 When an SMTP server returns a permanent error status (5yz) code after 2634 the DATA command is completed with ., it MUST NOT make 2635 any subsequent attempt to deliver the message. As with temporary 2636 error status codes, the SMTP client retains responsibility for the 2637 message, but SHOULD NOT again attempt delivery to the same server 2638 without user review of the message and response and appropriate 2639 intervention. 2641 4.3. Sequencing of Commands and Replies 2643 4.3.1. Sequencing Overview 2645 The communication between the sender and receiver is an alternating 2646 dialogue, controlled by the sender. As such, the sender issues a 2647 command and the receiver responds with a reply. Unless other 2648 arrangements are negotiated through service extensions, the sender 2649 MUST wait for this response before sending further commands. One 2650 important reply is the connection greeting. Normally, a receiver 2651 will send a 220 "Service ready" reply when the connection is 2652 completed. The sender SHOULD wait for this greeting message before 2653 sending any commands. 2655 Note: all the greeting-type replies have the official name (the 2656 fully-qualified primary domain name) of the server host as the first 2657 word following the reply code. Sometimes the host will have no 2658 meaningful name. See Section 4.1.3 for a discussion of alternatives 2659 in these situations. 2661 For example, 2662 220 ISIF.USC.EDU Service ready 2664 or 2666 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2668 or 2670 220 [10.0.0.1] Clueless host service ready 2672 The table below lists alternative success and failure replies for 2673 each command. These SHOULD be strictly adhered to. A receiver MAY 2674 substitute text in the replies, but the meanings and actions implied 2675 by the code numbers and by the specific command reply sequence MUST 2676 be preserved. However, in order to provide robustness as SMTP is 2677 extended and evolves, the discussion in Section 4.2.1 still applies: 2678 all SMTP clients MUST be prepared to accept any code that conforms to 2679 the discussion in that section and MUST be prepared to interpret it 2680 on the basis of its first digit only. [[CREF22: [5321bis] 20140914: 2681 Above sentence is new text based on yet another round of discussions 2682 about "invalid codes".]] 2684 4.3.2. Command-Reply Sequences 2686 Each command is listed with its usual possible replies. The prefixes 2687 used before the possible replies are "I" for intermediate, "S" for 2688 success, and "E" for error. Since some servers may generate other 2689 replies under special circumstances, and to allow for future 2690 extension, SMTP clients SHOULD, when possible, interpret only the 2691 first digit of the reply and MUST be prepared to deal with 2692 unrecognized reply codes by interpreting the first digit only. 2693 Unless extended using the mechanisms described in Section 2.2, SMTP 2694 servers MUST NOT transmit reply codes to an SMTP client that are 2695 other than three digits or that do not start in a digit between 2 and 2696 5 inclusive. 2698 These sequencing rules and, in principle, the codes themselves, can 2699 be extended or modified by SMTP extensions offered by the server and 2700 accepted (requested) by the client. However, if the target is more 2701 precise granularity in the codes, rather than codes for completely 2702 new purposes, the system described in RFC 3463 [34] SHOULD be used in 2703 preference to the invention of new codes. 2705 In addition to the codes listed below, any SMTP command can return 2706 any of the following codes if the corresponding unusual circumstances 2707 are encountered: 2709 500 For the "command line too long" case or if the command name was 2710 not recognized. Note that producing a "command not recognized" 2711 error in response to the required subset of these commands is a 2712 violation of this specification. Similarly, producing a "command 2713 too long" message for a command line shorter than 512 characters 2714 would violate the provisions of Section 4.5.3.1.4. 2716 501 Syntax error in command or arguments. In order to provide for 2717 future extensions, commands that are specified in this document as 2718 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2719 message if arguments are supplied in the absence of EHLO- 2720 advertised extensions. 2722 421 Service shutting down and closing transmission channel 2724 Specific sequences are: 2726 CONNECTION ESTABLISHMENT 2728 S: 220 2729 E: 521, 554 2731 EHLO or HELO 2733 S: 250 2734 E: 504 (a conforming implementation could return this code only 2735 in fairly obscure cases), 550, 502 (permitted only with an old- 2736 style server that does not support EHLO) 2738 MAIL 2740 S: 250 2741 E: 552, 451, 452, 550, 553, 503, 455, 555 2743 RCPT 2745 S: 250, 251 (but see Section 3.4 for discussion of 251 and 551) 2746 E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555 2748 DATA 2750 I: 354 -> data -> S: 250 2752 E: 552, 554, 451, 452 2754 E: 450, 550 (rejections for policy reasons) 2756 E: 503, 554 2758 RSET 2760 S: 250 2762 VRFY 2764 S: 250, 251, 252 2765 E: 550, 551, 553, 502, 504 2767 EXPN 2769 S: 250, 252 2770 E: 550, 500, 502, 504 2772 HELP 2774 S: 211, 214 2775 E: 502, 504 2777 NOOP 2779 S: 250 2781 QUIT 2783 S: 221 2785 4.4. Trace Information 2787 When an SMTP server receives a message for delivery or further 2788 processing, it MUST insert trace (often referred to as "time stamp" 2789 or "Received" information) [[CREF23: [5321bis] See note on 2790 rfc5321bis-00c above]] information at the beginning of the message 2791 content, as discussed in Section 4.1.1.4. 2793 This line MUST be structured as follows: 2795 o The FROM clause, which MUST be supplied in an SMTP environment, 2796 SHOULD contain both (1) the name of the source host as presented 2797 in the EHLO command and (2) an address literal containing the IP 2798 address of the source, determined from the TCP connection. 2800 o The ID clause MAY contain an "@" as suggested in RFC 822, but this 2801 is not required. 2803 o If the FOR clause appears, it MUST contain exactly one 2804 entry, even when multiple RCPT commands have been given. Multiple 2805 s raise some security issues and have been deprecated, see 2806 Section 7.2. 2808 An Internet mail program MUST NOT change or delete a Received: line 2809 that was previously added to the message header section. SMTP 2810 servers MUST prepend Received lines to messages; they MUST NOT change 2811 the order of existing lines or insert Received lines in any other 2812 location. 2814 As the Internet grows, comparability of Received header fields is 2815 important for detecting problems, especially slow relays. SMTP 2816 servers that create Received header fields SHOULD use explicit 2817 offsets in the dates (e.g., -0800), rather than time zone names of 2818 any type. Local time (with an offset) SHOULD be used rather than UT 2819 when feasible. This formulation allows slightly more information 2820 about local circumstances to be specified. If UT is needed, the 2821 receiver need merely do some simple arithmetic to convert the values. 2822 Use of UT loses information about the time zone-location of the 2823 server. If it is desired to supply a time zone name, it SHOULD be 2824 included in a comment. 2826 When the delivery SMTP server makes the "final delivery" of a 2827 message, it inserts a return-path line at the beginning of the mail 2828 data. This use of return-path is required; mail systems MUST support 2829 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2831 has left the SMTP environment. Normally, this would mean it had been 2832 delivered to the destination user or an associated mail drop, but in 2833 some cases it may be further processed and transmitted by another 2834 mail system. 2836 It is possible for the mailbox in the return path to be different 2837 from the actual sender's mailbox, for example, if error responses are 2838 to be delivered to a special error handling mailbox rather than to 2839 the message sender. When mailing lists are involved, this 2840 arrangement is common and useful as a means of directing errors to 2841 the list maintainer rather than the message originator. 2843 The text above implies that the final mail data will begin with a 2844 return path line, followed by one or more time stamp lines. These 2845 lines will be followed by the rest of the mail data: first the 2846 balance of the mail header section and then the body (RFC 5322 [11]). 2848 It is sometimes difficult for an SMTP server to determine whether or 2849 not it is making final delivery since forwarding or other operations 2850 may occur after the message is accepted for delivery. Consequently, 2851 any further (forwarding, gateway, or relay) systems MAY remove the 2852 return path and rebuild the MAIL command as needed to ensure that 2853 exactly one such line appears in a delivered message. 2855 A message-originating SMTP system SHOULD NOT send a message that 2856 already contains a Return-path header field. SMTP servers performing 2857 a relay function MUST NOT inspect the message data, and especially 2858 not to the extent needed to determine if Return-path header fields 2859 are present. SMTP servers making final delivery MAY remove Return- 2860 path header fields before adding their own. 2862 The primary purpose of the Return-path is to designate the address to 2863 which messages indicating non-delivery or other mail system failures 2864 are to be sent. For this to be unambiguous, exactly one return path 2865 SHOULD be present when the message is delivered. Systems using RFC 2866 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2867 address, associated with the transport envelope, to which error 2868 reports (e.g., non-delivery messages) should be sent. 2870 Historical note: Text in RFC 822 that appears to contradict the use 2871 of the Return-path header field (or the envelope reverse-path address 2872 from the MAIL command) as the destination for error messages is not 2873 applicable on the Internet. The reverse-path address (as copied into 2874 the Return-path) MUST be used as the target of any mail containing 2875 delivery error messages. 2877 In particular: 2879 o a gateway from SMTP -> elsewhere SHOULD insert a return-path 2880 header field, unless it is known that the "elsewhere" transport 2881 also uses Internet domain addresses and maintains the envelope 2882 sender address separately. 2884 o a gateway from elsewhere -> SMTP SHOULD delete any return-path 2885 header field present in the message, and either copy that 2886 information to the SMTP envelope or combine it with information 2887 present in the envelope of the other transport system to construct 2888 the reverse-path argument to the MAIL command in the SMTP 2889 envelope. 2891 The server must give special treatment to cases in which the 2892 processing following the end of mail data indication is only 2893 partially successful. This could happen if, after accepting several 2894 recipients and the mail data, the SMTP server finds that the mail 2895 data could be successfully delivered to some, but not all, of the 2896 recipients. In such cases, the response to the DATA command MUST be 2897 an OK reply. However, the SMTP server MUST compose and send an 2898 "undeliverable mail" notification message to the originator of the 2899 message. 2901 A single notification listing all of the failed recipients or 2902 separate notification messages MUST be sent for each failed 2903 recipient. For economy of processing by the sender, the former 2904 SHOULD be used when possible. Note that the key difference between 2905 handling aliases (Section 3.9.1) and forwarding (this subsection) is 2906 the change to the backward-pointing address in this case. All 2907 notification messages about undeliverable mail MUST be sent using the 2908 MAIL command (even if they result from processing the obsolete SEND, 2909 SOML, or SAML commands) and MUST use a null return path as discussed 2910 in Section 3.6. 2912 The time stamp line and the return path line are formally defined as 2913 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2914 [11]): 2916 Return-path-line = "Return-Path:" FWS Reverse-path 2918 Time-stamp-line = "Received:" FWS Stamp 2920 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2921 FWS date-time 2922 ; where "date-time" is as defined in RFC 5322 [11] 2923 ; but the "obs-" forms, especially two-digit 2924 ; years, are prohibited in SMTP and MUST NOT be used. 2926 From-domain = "FROM" FWS Extended-Domain 2928 By-domain = CFWS "BY" FWS Extended-Domain 2930 Extended-Domain = Domain / 2931 ( Domain FWS "(" TCP-info ")" ) / 2932 ( address-literal FWS "(" TCP-info ")" ) 2934 TCP-info = address-literal / ( Domain FWS address-literal ) 2935 ; Information derived by server from TCP connection 2936 ; not client EHLO. 2938 Opt-info = [Via] [With] [ID] [For] 2939 [Additional-Registered-Clauses] 2941 Via = CFWS "VIA" FWS Link 2943 With = CFWS "WITH" FWS Protocol 2944 ID = CFWS "ID" FWS ( Atom / msg-id ) 2945 ; msg-id is defined in RFC 5322 [11] 2947 For = CFWS "FOR" FWS ( Path / Mailbox ) 2949 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 2950 [[CREF24: [5321bis] 5321 errata #1683, 20090215, 2951 Roberto Javier Godoy, rjgodoy@fich.unl.edu.ar]] 2952 ; Additional standard clauses may be added in this 2953 ; location by future standards and registration with 2954 ; IANA. SMTP servers SHOULD NOT use unregistered 2955 ; names. See Section 8. 2957 Link = "TCP" / Addtl-Link 2959 Addtl-Link = Atom 2960 ; Additional standard names for links are 2961 ; registered with the Internet Assigned Numbers 2962 ; Authority (IANA). "Via" is primarily of value 2963 ; with non-Internet transports. SMTP servers 2964 ; SHOULD NOT use unregistered names. 2966 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 2968 Addtl-Protocol = Atom 2969 ; Additional standard names for protocols are 2970 ; registered with the Internet Assigned Numbers 2971 ; Authority (IANA) in the "mail parameters" 2972 ; registry [7]. SMTP servers SHOULD NOT 2973 ; use unregistered names. 2975 4.5. Additional Implementation Issues 2977 4.5.1. Minimum Implementation 2979 In order to make SMTP workable, the following minimum implementation 2980 MUST be provided by all receivers. The following commands MUST be 2981 supported to conform to this specification: 2983 EHLO 2984 HELO 2985 MAIL 2986 RCPT 2987 DATA 2988 RSET 2989 NOOP 2990 QUIT 2991 VRFY 2993 Any system that includes an SMTP server supporting mail relaying or 2994 delivery MUST support the reserved mailbox "postmaster" as a case- 2995 insensitive local name. This postmaster address is not strictly 2996 necessary if the server always returns 554 on connection opening (as 2997 described in Section 3.1). The requirement to accept mail for 2998 postmaster implies that RCPT commands that specify a mailbox for 2999 postmaster at any of the domains for which the SMTP server provides 3000 mail service, as well as the special case of "RCPT TO:" 3001 (with no domain specification), MUST be supported. 3003 SMTP systems are expected to make every reasonable effort to accept 3004 mail directed to Postmaster from any other system on the Internet. 3005 In extreme cases -- such as to contain a denial of service attack or 3006 other breach of security -- an SMTP server may block mail directed to 3007 Postmaster. However, such arrangements SHOULD be narrowly tailored 3008 so as to avoid blocking messages that are not part of such attacks. 3010 4.5.2. Transparency 3012 Without some provision for data transparency, the character sequence 3013 "." ends the mail text and cannot be sent by the user. 3014 In general, users are not aware of such "forbidden" sequences. To 3015 allow all user composed text to be transmitted transparently, the 3016 following procedures are used: 3018 o Before sending a line of mail text, the SMTP client checks the 3019 first character of the line. If it is a period, one additional 3020 period is inserted at the beginning of the line. 3022 o When a line of mail text is received by the SMTP server, it checks 3023 the line. If the line is composed of a single period, it is 3024 treated as the end of mail indicator. If the first character is a 3025 period and there are other characters on the line, the first 3026 character is deleted. 3028 The mail data may contain any of the 128 ASCII characters. All 3029 characters are to be delivered to the recipient's mailbox, including 3030 spaces, vertical and horizontal tabs, and other control characters. 3031 If the transmission channel provides an 8-bit byte (octet) data 3032 stream, the 7-bit ASCII codes are transmitted, right justified, in 3033 the octets, with the high-order bits cleared to zero. See 3034 Section 3.6 for special treatment of these conditions in SMTP systems 3035 serving a relay function. 3037 In some systems, it may be necessary to transform the data as it is 3038 received and stored. This may be necessary for hosts that use a 3039 different character set than ASCII as their local character set, that 3040 store data in records rather than strings, or which use special 3041 character sequences as delimiters inside mailboxes. If such 3042 transformations are necessary, they MUST be reversible, especially if 3043 they are applied to mail being relayed. 3045 4.5.3. Sizes and Timeouts 3047 4.5.3.1. Size Limits and Minimums 3049 There are several objects that have required minimum/maximum sizes. 3050 Every implementation MUST be able to receive objects of at least 3051 these sizes. Objects larger than these sizes SHOULD be avoided when 3052 possible. However, some Internet mail constructs such as encoded 3053 X.400 addresses (RFC 2156 [26]) will often require larger objects. 3054 Clients MAY attempt to transmit these, but MUST be prepared for a 3055 server to reject them if they cannot be handled by it. To the 3056 maximum extent possible, implementation techniques that impose no 3057 limits on the length of these objects should be used. 3059 Extensions to SMTP may involve the use of characters that occupy more 3060 than a single octet each. This section therefore specifies lengths 3061 in octets where absolute lengths, rather than character counts, are 3062 intended. 3064 [[CREF25: [5321bis] [[Note in Draft: Klensin 20191126: Given the 3065 controversy on the SMTP mailing list between 20191123 and now about 3066 maximum lengths, is the above adequate or is further tuning of the 3067 limit text below needed? ]]]] 3069 4.5.3.1.1. Local-part 3071 The maximum total length of a user name or other local-part is 64 3072 octets. 3074 4.5.3.1.2. Domain 3076 The maximum total length of a domain name or number is 255 octets. 3078 4.5.3.1.3. Path 3080 The maximum total length of a reverse-path or forward-path is 256 3081 octets (including the punctuation and element separators). 3083 4.5.3.1.4. Command Line 3085 The maximum total length of a command line including the command word 3086 and the is 512 octets. SMTP extensions may be used to 3087 increase this limit. 3089 4.5.3.1.5. Reply Line 3091 The maximum total length of a reply line including the reply code and 3092 the is 512 octets. More information may be conveyed through 3093 multiple-line replies. 3095 4.5.3.1.6. Text Line 3097 The maximum total length of a text line including the is 1000 3098 octets (not counting the leading dot duplicated for transparency). 3099 This number may be increased by the use of SMTP Service Extensions. 3101 4.5.3.1.7. Message Content 3103 The maximum total length of a message content (including any message 3104 header section as well as the message body) MUST BE at least 64K 3105 octets. Since the introduction of Internet Standards for multimedia 3106 mail (RFC 2045 [24]), message lengths on the Internet have grown 3107 dramatically, and message size restrictions should be avoided if at 3108 all possible. SMTP server systems that must impose restrictions 3109 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3110 SMTP client systems that will send large messages SHOULD utilize it 3111 when possible. 3113 4.5.3.1.8. Recipient Buffer 3115 The minimum total number of recipients that MUST be buffered is 100 3116 recipients. Rejection of messages (for excessive recipients) with 3117 fewer than 100 RCPT commands is a violation of this specification. 3118 The general principle that relaying SMTP server MUST NOT, and 3119 delivery SMTP servers SHOULD NOT, perform validation tests on message 3120 header fields suggests that messages SHOULD NOT be rejected based on 3121 the total number of recipients shown in header fields. A server that 3122 imposes a limit on the number of recipients MUST behave in an orderly 3123 fashion, such as rejecting additional addresses over its limit rather 3124 than silently discarding addresses previously accepted. A client 3125 that needs to deliver a message containing over 100 RCPT commands 3126 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3127 server declines to accept more than 100 recipients in a single 3128 message. 3130 4.5.3.1.9. Treatment When Limits Exceeded 3132 Errors due to exceeding these limits may be reported by using the 3133 reply codes. Some examples of reply codes are: 3135 500 Line too long. 3137 or 3139 501 Path too long 3141 or 3143 452 Too many recipients (see below) 3145 or 3147 552 Too much mail data. 3149 4.5.3.1.10. Too Many Recipients Code 3151 RFC 821 [3] incorrectly listed the error where an SMTP server 3152 exhausts its implementation limit on the number of RCPT commands 3153 ("too many recipients") as having reply code 552. The correct reply 3154 code for this condition is 452. Clients SHOULD treat a 552 code in 3155 this case as a temporary, rather than permanent, failure so the logic 3156 below works. 3158 When a conforming SMTP server encounters this condition, it has at 3159 least 100 successful RCPT commands in its recipient buffer. If the 3160 server is able to accept the message, then at least these 100 3161 addresses will be removed from the SMTP client's queue. When the 3162 client attempts retransmission of those addresses that received 452 3163 responses, at least 100 of these will be able to fit in the SMTP 3164 server's recipient buffer. Each retransmission attempt that is able 3165 to deliver anything will be able to dispose of at least 100 of these 3166 recipients. 3168 If an SMTP server has an implementation limit on the number of RCPT 3169 commands and this limit is exhausted, it MUST use a response code of 3170 452 (but the client SHOULD also be prepared for a 552, as noted 3171 above). If the server has a configured site-policy limitation on the 3172 number of RCPT commands, it MAY instead use a 5yz response code. In 3173 particular, if the intent is to prohibit messages with more than a 3174 site-specified number of recipients, rather than merely limit the 3175 number of recipients in a given mail transaction, it would be 3176 reasonable to return a 503 response to any DATA command received 3177 subsequent to the 452 (or 552) code or to simply return the 503 after 3178 DATA without returning any previous negative response. 3180 4.5.3.2. Timeouts 3182 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3183 command timeouts rather than somehow trying to time the entire mail 3184 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3185 without recompiling the SMTP code. To implement this, a timer is set 3186 for each SMTP command and for each buffer of the data transfer. The 3187 latter means that the overall timeout is inherently proportional to 3188 the size of the message. 3190 Based on extensive experience with busy mail-relay hosts, the minimum 3191 per-command timeout values SHOULD be as follows: 3193 4.5.3.2.1. Initial 220 Message: 5 Minutes 3195 An SMTP client process needs to distinguish between a failed TCP 3196 connection and a delay in receiving the initial 220 greeting message. 3197 Many SMTP servers accept a TCP connection but delay delivery of the 3198 220 message until their system load permits more mail to be 3199 processed. 3201 4.5.3.2.2. MAIL Command: 5 Minutes 3203 4.5.3.2.3. RCPT Command: 5 Minutes 3205 A longer timeout is required if processing of mailing lists and 3206 aliases is not deferred until after the message was accepted. 3208 4.5.3.2.4. DATA Initiation: 2 Minutes 3210 This is while awaiting the "354 Start Input" reply to a DATA command. 3212 4.5.3.2.5. Data Block: 3 Minutes 3214 This is while awaiting the completion of each TCP SEND call 3215 transmitting a chunk of data. 3217 4.5.3.2.6. DATA Termination: 10 Minutes. 3219 This is while awaiting the "250 OK" reply. When the receiver gets 3220 the final period terminating the message data, it typically performs 3221 processing to deliver the message to a user mailbox. A spurious 3222 timeout at this point would be very wasteful and would typically 3223 result in delivery of multiple copies of the message, since it has 3224 been successfully sent and the server has accepted responsibility for 3225 delivery. See Section 6.1 for additional discussion. 3227 4.5.3.2.7. Server Timeout: 5 Minutes. 3229 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3230 is awaiting the next command from the sender. 3232 4.5.4. Retry Strategies 3234 The common structure of a host SMTP implementation includes user 3235 mailboxes, one or more areas for queuing messages in transit, and one 3236 or more daemon processes for sending and receiving mail. The exact 3237 structure will vary depending on the needs of the users on the host 3238 and the number and size of mailing lists supported by the host. We 3239 describe several optimizations that have proved helpful, particularly 3240 for mailers supporting high traffic levels. 3242 Any queuing strategy MUST include timeouts on all activities on a 3243 per-command basis. A queuing strategy MUST NOT send error messages 3244 in response to error messages under any circumstances. 3246 4.5.4.1. Sending Strategy 3248 The general model for an SMTP client is one or more processes that 3249 periodically attempt to transmit outgoing mail. In a typical system, 3250 the program that composes a message has some method for requesting 3251 immediate attention for a new piece of outgoing mail, while mail that 3252 cannot be transmitted immediately MUST be queued and periodically 3253 retried by the sender. A mail queue entry will include not only the 3254 message itself but also the envelope information. 3256 The sender MUST delay retrying a particular destination after one 3257 attempt has failed. In general, the retry interval SHOULD be at 3258 least 30 minutes; however, more sophisticated and variable strategies 3259 will be beneficial when the SMTP client can determine the reason for 3260 non-delivery. 3262 Retries continue until the message is transmitted or the sender gives 3263 up; the give-up time generally needs to be at least 4-5 days. It MAY 3264 be appropriate to set a shorter maximum number of retries for non- 3265 delivery notifications and equivalent error messages than for 3266 standard messages. The parameters to the retry algorithm MUST be 3267 configurable. 3269 A client SHOULD keep a list of hosts it cannot reach and 3270 corresponding connection timeouts, rather than just retrying queued 3271 mail items. 3273 Experience suggests that failures are typically transient (the target 3274 system or its connection has crashed), favoring a policy of two 3275 connection attempts in the first hour the message is in the queue, 3276 and then backing off to one every two or three hours. 3278 The SMTP client can shorten the queuing delay in cooperation with the 3279 SMTP server. For example, if mail is received from a particular 3280 address, it is likely that mail queued for that host can now be sent. 3281 Application of this principle may, in many cases, eliminate the 3282 requirement for an explicit "send queues now" function such as ETRN, 3283 RFC 1985 [23]. 3285 The strategy may be further modified as a result of multiple 3286 addresses per host (see below) to optimize delivery time versus 3287 resource usage. 3289 An SMTP client may have a large queue of messages for each 3290 unavailable destination host. If all of these messages were retried 3291 in every retry cycle, there would be excessive Internet overhead and 3292 the sending system would be blocked for a long period. Note that an 3293 SMTP client can generally determine that a delivery attempt has 3294 failed only after a timeout of several minutes, and even a one-minute 3295 timeout per connection will result in a very large delay if retries 3296 are repeated for dozens, or even hundreds, of queued messages to the 3297 same host. 3299 At the same time, SMTP clients SHOULD use great care in caching 3300 negative responses from servers. In an extreme case, if EHLO is 3301 issued multiple times during the same SMTP connection, different 3302 answers may be returned by the server. More significantly, 5yz 3303 responses to the MAIL command MUST NOT be cached. 3305 When a mail message is to be delivered to multiple recipients, and 3306 the SMTP server to which a copy of the message is to be sent is the 3307 same for multiple recipients, then only one copy of the message 3308 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3309 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3310 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3311 are very many addresses, a limit on the number of RCPT commands per 3312 MAIL command MAY be imposed. This efficiency feature SHOULD be 3313 implemented. 3315 Similarly, to achieve timely delivery, the SMTP client MAY support 3316 multiple concurrent outgoing mail transactions. However, some limit 3317 may be appropriate to protect the host from devoting all its 3318 resources to mail. 3320 4.5.4.2. Receiving Strategy 3322 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3323 port (specified by IANA as port 25) at all times. This requires the 3324 support of multiple incoming TCP connections for SMTP. Some limit 3325 MAY be imposed, but servers that cannot handle more than one SMTP 3326 transaction at a time are not in conformance with the intent of this 3327 specification. 3329 As discussed above, when the SMTP server receives mail from a 3330 particular host address, it could activate its own SMTP queuing 3331 mechanisms to retry any mail pending for that host address. 3333 4.5.5. Messages with a Null Reverse-Path 3335 There are several types of notification messages that are required by 3336 existing and proposed Standards to be sent with a null reverse-path, 3337 namely non-delivery notifications as discussed in Section 3.7, other 3338 kinds of Delivery Status Notifications (DSNs, RFC 3461 [33]), and 3339 Message Disposition Notifications (MDNs, RFC 3798 [37]). All of 3340 these kinds of messages are notifications about a previous message, 3341 and they are sent to the reverse-path of the previous mail message. 3342 (If the delivery of such a notification message fails, that usually 3343 indicates a problem with the mail system of the host to which the 3344 notification message is addressed. For this reason, at some hosts 3345 the MTA is set up to forward such failed notification messages to 3346 someone who is able to fix problems with the mail system, e.g., via 3347 the postmaster alias.) 3349 All other types of messages (i.e., any message which is not required 3350 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3351 with a valid, non-null reverse-path. 3353 Implementers of automated email processors should be careful to make 3354 sure that the various kinds of messages with a null reverse-path are 3355 handled correctly. In particular, such systems SHOULD NOT reply to 3356 messages with a null reverse-path, and they SHOULD NOT add a non-null 3357 reverse-path, or change a null reverse-path to a non-null one, to 3358 such messages when forwarding. 3360 5. Address Resolution and Mail Handling 3362 5.1. Locating the Target Host 3364 Once an SMTP client lexically identifies a domain to which mail will 3365 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3366 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3367 [4]). The names are expected to be fully-qualified domain names 3368 (FQDNs): mechanisms for inferring FQDNs from partial names or local 3369 aliases are outside of this specification. Due to a history of 3370 problems, SMTP servers used for initial submission of messages SHOULD 3371 NOT make such inferences (Message Submission Servers [42] have 3372 somewhat more flexibility) and intermediate (relay) SMTP servers MUST 3373 NOT make them. 3375 The lookup first attempts to locate an MX record associated with the 3376 name. If a CNAME record is found, the resulting name is processed as 3377 if it were the initial name. If a non-existent domain error is 3378 returned, this situation MUST be reported as an error. If a 3379 temporary error is returned, the message MUST be queued and retried 3380 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3381 the address is treated as if it was associated with an implicit MX 3382 RR, with a preference of 0, pointing to that host. If MX records are 3383 present, but none of them are usable, or the implicit MX is unusable, 3384 this situation MUST be reported as an error. 3386 If one or more MX RRs are found for a given name, SMTP systems MUST 3387 NOT utilize any address RRs associated with that name unless they are 3388 located using the MX RRs; the "implicit MX" rule above applies only 3389 if there are no MX records present. If MX records are present, but 3390 none of them are usable, this situation MUST be reported as an error. 3392 When a domain name associated with an MX RR is looked up and the 3393 associated data field obtained, the data field of that response MUST 3394 contain a domain name that conforms to the specifications of 3395 Section 2.3.5. 3396 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3397 unfolds, some additional text may be in order here (20140718)]] 3398 That domain name, when queried, MUST return at least one address 3399 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3400 server to which the message should be directed. Any other response, 3401 specifically including a value that will return a CNAME record when 3402 queried, lies outside the scope of this Standard. The prohibition on 3403 labels in the data that resolve to CNAMEs is discussed in more detail 3404 in RFC 2181, Section 10.3 [27]. 3406 When the lookup succeeds, the mapping can result in a list of 3407 alternative delivery addresses rather than a single address, because 3408 of multiple MX records, multihoming, or both. To provide reliable 3409 mail transmission, the SMTP client MUST be able to try (and retry) 3410 each of the relevant addresses in this list in order, until a 3411 delivery attempt succeeds. However, there MAY also be a configurable 3412 limit on the number of alternate addresses that can be tried. In any 3413 case, the SMTP client SHOULD try at least two addresses. 3415 Two types of information are used to rank the host addresses: 3416 multiple MX records, and multihomed hosts. 3418 MX records contain a preference indication that MUST be used in 3419 sorting if more than one such record appears (see below). Lower 3420 numbers are more preferred than higher ones. If there are multiple 3421 destinations with the same preference and there is no clear reason to 3422 favor one (e.g., by recognition of an easily reached address), then 3423 the sender-SMTP MUST randomize them to spread the load across 3424 multiple mail exchangers for a specific organization. 3426 The destination host (perhaps taken from the preferred MX record) may 3427 be multihomed, in which case the domain name resolver will return a 3428 list of alternative IP addresses. It is the responsibility of the 3429 domain name resolver interface to have ordered this list by 3430 decreasing preference if necessary, and the SMTP sender MUST try them 3431 in the order presented. 3433 Although the capability to try multiple alternative addresses is 3434 required, specific installations may want to limit or disable the use 3435 of alternative addresses. The question of whether a sender should 3436 attempt retries using the different addresses of a multihomed host 3437 has been controversial. The main argument for using the multiple 3438 addresses is that it maximizes the probability of timely delivery, 3439 and indeed sometimes the probability of any delivery; the counter- 3440 argument is that it may result in unnecessary resource use. Note 3441 that resource use is also strongly determined by the sending strategy 3442 discussed in Section 4.5.4.1. 3444 If an SMTP server receives a message with a destination for which it 3445 is a designated Mail eXchanger, it MAY relay the message (potentially 3446 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3447 final delivery of the message, or hand it off using some mechanism 3448 outside the SMTP-provided transport environment. Of course, neither 3449 of the latter require that the list of MX records be examined 3450 further. 3452 If it determines that it should relay the message without rewriting 3453 the address, it MUST sort the MX records to determine candidates for 3454 delivery. The records are first ordered by preference, with the 3455 lowest-numbered records being most preferred. The relay host MUST 3456 then inspect the list for any of the names or addresses by which it 3457 might be known in mail transactions. If a matching record is found, 3458 all records at that preference level and higher-numbered ones MUST be 3459 discarded from consideration. If there are no records left at that 3460 point, it is an error condition, and the message MUST be returned as 3461 undeliverable. If records do remain, they SHOULD be tried, best 3462 preference first, as described above. 3464 5.2. IPv6 and MX Records 3466 In the contemporary Internet, SMTP clients and servers may be hosted 3467 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3468 compatible with either version of the Internet Protocol. The host 3469 domains to which MX records point may, consequently, contain "A RR"s 3470 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3471 3974 [39] discusses some operational experience in mixed 3472 environments, it was not comprehensive enough to justify 3473 standardization, and some of its recommendations appear to be 3474 inconsistent with this specification. The appropriate actions to be 3475 taken either will depend on local circumstances, such as performance 3476 of the relevant networks and any conversions that might be necessary, 3477 or will be obvious (e.g., an IPv6-only client need not attempt to 3478 look up A RRs or attempt to reach IPv4-only servers). Designers of 3479 SMTP implementations that might run in IPv6 or dual-stack 3480 environments should study the procedures above, especially the 3481 comments about multihomed hosts, and, preferably, provide mechanisms 3482 to facilitate operational tuning and mail interoperability between 3483 IPv4 and IPv6 systems while considering local circumstances. 3485 6. Problem Detection and Handling 3487 6.1. Reliable Delivery and Replies by Email 3489 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3490 message in response to DATA), it is accepting responsibility for 3491 delivering or relaying the message. It must take this responsibility 3492 seriously. It MUST NOT lose the message for frivolous reasons, such 3493 as because the host later crashes or because of a predictable 3494 resource shortage. Some reasons that are not considered frivolous 3495 are discussed in the next subsection and in Section 7.8. 3497 If there is a delivery failure after acceptance of a message, the 3498 receiver-SMTP MUST formulate and mail a notification message. This 3499 notification MUST be sent using a null ("<>") reverse-path in the 3500 envelope. The recipient of this notification MUST be the address 3501 from the envelope return path (or the Return-Path: line). However, 3502 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3503 notification. Obviously, nothing in this section can or should 3504 prohibit local decisions (i.e., as part of the same system 3505 environment as the receiver-SMTP) to log or otherwise transmit 3506 information about null address events locally if that is desired. If 3507 the address is an explicit source route, it MUST be stripped down to 3508 its final hop. 3510 For example, suppose that an error notification must be sent for a 3511 message that arrived with: 3513 MAIL FROM:<@a,@b:user@d> 3515 The notification message MUST be sent using: 3517 RCPT TO: 3519 Some delivery failures after the message is accepted by SMTP will be 3520 unavoidable. For example, it may be impossible for the receiving 3521 SMTP server to validate all the delivery addresses in RCPT command(s) 3522 due to a "soft" domain system error, because the target is a mailing 3523 list (see earlier discussion of RCPT), or because the server is 3524 acting as a relay and has no immediate access to the delivering 3525 system. 3527 To avoid receiving duplicate messages as the result of timeouts, a 3528 receiver-SMTP MUST seek to minimize the time required to respond to 3529 the final . end of data indicator. See RFC 1047 [16] for 3530 a discussion of this problem. 3532 6.2. Unwanted, Unsolicited, and "Attack" Messages 3534 Utility and predictability of the Internet mail system requires that 3535 messages that can be delivered should be delivered, regardless of any 3536 syntax or other faults associated with those messages and regardless 3537 of their content. If they cannot be delivered, and cannot be 3538 rejected by the SMTP server during the SMTP transaction, they should 3539 be "bounced" (returned with non-delivery notification messages) as 3540 described above. In today's world, in which many SMTP server 3541 operators have discovered that the quantity of undesirable bulk email 3542 vastly exceeds the quantity of desired mail and in which accepting a 3543 message may trigger additional undesirable traffic by providing 3544 verification of the address, those principles may not be practical. 3546 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3547 without notification of the sender is permitted in practice. 3548 However, it is extremely dangerous and violates a long tradition and 3549 community expectations that mail is either delivered or returned. If 3550 silent message-dropping is misused, it could easily undermine 3551 confidence in the reliability of the Internet's mail systems. So 3552 silent dropping of messages should be considered only in those cases 3553 where there is very high confidence that the messages are seriously 3554 fraudulent or otherwise inappropriate. 3556 To stretch the principle of delivery if possible even further, it may 3557 be a rational policy to not deliver mail that has an invalid return 3558 address, although the history of the network is that users are 3559 typically better served by delivering any message that can be 3560 delivered. Reliably determining that a return address is invalid can 3561 be a difficult and time-consuming process, especially if the putative 3562 sending system is not directly accessible or does not fully and 3563 accurately support VRFY and, even if a "drop messages with invalid 3564 return addresses" policy is adopted, it SHOULD be applied only when 3565 there is near-certainty that the return addresses are, in fact, 3566 invalid. 3568 Conversely, if a message is rejected because it is found to contain 3569 hostile content (a decision that is outside the scope of an SMTP 3570 server as defined in this document), rejection ("bounce") messages 3571 SHOULD NOT be sent unless the receiving site is confident that those 3572 messages will be usefully delivered. The preference and default in 3573 these cases is to avoid sending non-delivery messages when the 3574 incoming message is determined to contain hostile content. 3576 6.3. Loop Detection 3578 Simple counting of the number of "Received:" header fields in a 3579 message has proven to be an effective, although rarely optimal, 3580 method of detecting loops in mail systems. SMTP servers using this 3581 technique SHOULD use a large rejection threshold, normally at least 3582 100 Received entries. Whatever mechanisms are used, servers MUST 3583 contain provisions for detecting and stopping trivial loops. 3585 6.4. Compensating for Irregularities 3587 Unfortunately, variations, creative interpretations, and outright 3588 violations of Internet mail protocols do occur; some would suggest 3589 that they occur quite frequently. The debate as to whether a well- 3590 behaved SMTP receiver or relay should reject a malformed message, 3591 attempt to pass it on unchanged, or attempt to repair it to increase 3592 the odds of successful delivery (or subsequent reply) began almost 3593 with the dawn of structured network mail and shows no signs of 3594 abating. Advocates of rejection claim that attempted repairs are 3595 rarely completely adequate and that rejection of bad messages is the 3596 only way to get the offending software repaired. Advocates of 3597 "repair" or "deliver no matter what" argue that users prefer that 3598 mail go through it if at all possible and that there are significant 3599 market pressures in that direction. In practice, these market 3600 pressures may be more important to particular vendors than strict 3601 conformance to the standards, regardless of the preference of the 3602 actual developers. 3604 The problems associated with ill-formed messages were exacerbated by 3605 the introduction of the split-UA mail reading protocols (Post Office 3606 Protocol (POP) version 2 [13], Post Office Protocol (POP) version 3 3607 [22], IMAP version 2 [18], and PCMAIL [17]). These protocols 3608 encouraged the use of SMTP as a posting (message submission) 3609 protocol, and SMTP servers as relay systems for these client hosts 3610 (which are often only intermittently connected to the Internet). 3611 Historically, many of those client machines lacked some of the 3612 mechanisms and information assumed by SMTP (and indeed, by the mail 3613 format protocol, RFC 822 [12]). Some could not keep adequate track 3614 of time; others had no concept of time zones; still others could not 3615 identify their own names or addresses; and, of course, none could 3616 satisfy the assumptions that underlay RFC 822's conception of 3617 authenticated addresses. 3619 In response to these weak SMTP clients, many SMTP systems now 3620 complete messages that are delivered to them in incomplete or 3621 incorrect form. This strategy is generally considered appropriate 3622 when the server can identify or authenticate the client, and there 3623 are prior agreements between them. By contrast, there is at best 3624 great concern about fixes applied by a relay or delivery SMTP server 3625 that has little or no knowledge of the user or client machine. Many 3626 of these issues are addressed by using a separate protocol, such as 3627 that defined in RFC 4409 [42], for message submission, rather than 3628 using originating SMTP servers for that purpose. 3630 The following changes to a message being processed MAY be applied 3631 when necessary by an originating SMTP server, or one used as the 3632 target of SMTP as an initial posting (message submission) protocol: 3634 o Addition of a message-id field when none appears 3636 o Addition of a date, time, or time zone when none appears 3638 o Correction of addresses to proper FQDN format 3640 The less information the server has about the client, the less likely 3641 these changes are to be correct and the more caution and conservatism 3642 should be applied when considering whether or not to perform fixes 3643 and how. These changes MUST NOT be applied by an SMTP server that 3644 provides an intermediate relay function. 3646 In all cases, properly operating clients supplying correct 3647 information are preferred to corrections by the SMTP server. In all 3648 cases, documentation SHOULD be provided in trace header fields and/or 3649 header field comments for actions performed by the servers. 3651 7. Security Considerations 3653 7.1. Mail Security and Spoofing 3655 SMTP mail is inherently insecure in that it is feasible for even 3656 fairly casual users to negotiate directly with receiving and relaying 3657 SMTP servers and create messages that will trick a naive recipient 3658 into believing that they came from somewhere else. Constructing such 3659 a message so that the "spoofed" behavior cannot be detected by an 3660 expert is somewhat more difficult, but not sufficiently so as to be a 3661 deterrent to someone who is determined and knowledgeable. 3662 Consequently, as knowledge of Internet mail increases, so does the 3663 knowledge that SMTP mail inherently cannot be authenticated, or 3664 integrity checks provided, at the transport level. Real mail 3665 security lies only in end-to-end methods involving the message 3666 bodies, such as those that use digital signatures (see RFC 1847 [20] 3667 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [45] or Secure/ 3668 Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [38]). 3670 Various protocol extensions and configuration options that provide 3671 authentication at the transport level (e.g., from an SMTP client to 3672 an SMTP server) improve somewhat on the traditional situation 3673 described above. However, in general, they only authenticate one 3674 server to another rather than a chain of relays and servers, much 3675 less authenticating users or user machines. Consequently, unless 3676 they are accompanied by careful handoffs of responsibility in a 3677 carefully designed trust environment, they remain inherently weaker 3678 than end-to-end mechanisms that use digitally signed messages rather 3679 than depending on the integrity of the transport system. 3681 Efforts to make it more difficult for users to set envelope return 3682 path and header "From" fields to point to valid addresses other than 3683 their own are largely misguided: they frustrate legitimate 3684 applications in which mail is sent by one user on behalf of another, 3685 in which error (or normal) replies should be directed to a special 3686 address, or in which a single message is sent to multiple recipients 3687 on different hosts. (Systems that provide convenient ways for users 3688 to alter these header fields on a per-message basis should attempt to 3689 establish a primary and permanent mailbox address for the user so 3690 that Sender header fields within the message data can be generated 3691 sensibly.) 3693 This specification does not further address the authentication issues 3694 associated with SMTP other than to advocate that useful functionality 3695 not be disabled in the hope of providing some small margin of 3696 protection against a user who is trying to fake mail. 3698 7.2. "Blind" Copies 3700 Addresses that do not appear in the message header section may appear 3701 in the RCPT commands to an SMTP server for a number of reasons. The 3702 two most common involve the use of a mailing address as a "list 3703 exploder" (a single address that resolves into multiple addresses) 3704 and the appearance of "blind copies". Especially when more than one 3705 RCPT command is present, and in order to avoid defeating some of the 3706 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3707 the full set of RCPT command arguments into the header section, 3708 either as part of trace header fields or as informational or private- 3709 extension header fields. [[CREF26: [rfc5321bis] [[Note in draft - 3710 Suggestion from 20070124 that got lost: delete "especially" and "the 3711 full set of" -- copying the first one can be as harmful as copying 3712 all of them, at least without verifying that the addresses do appear 3713 in the headers.]] Arnt Gulbrandsen, arnt@oryx.com, 2007.01.24 3714 1121+0100]] Since this rule is often violated in practice, and cannot 3715 be enforced, sending SMTP systems that are aware of "bcc" use MAY 3716 find it helpful to send each blind copy as a separate message 3717 transaction containing only a single RCPT command. 3719 There is no inherent relationship between either "reverse" (from 3720 MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP 3721 transaction ("envelope") and the addresses in the header section. 3722 Receiving systems SHOULD NOT attempt to deduce such relationships and 3723 use them to alter the header section of the message for delivery. 3724 The popular "Apparently-to" header field is a violation of this 3725 principle as well as a common source of unintended information 3726 disclosure and SHOULD NOT be used. 3728 7.3. VRFY, EXPN, and Security 3730 As discussed in Section 3.5, individual sites may want to disable 3731 either or both of VRFY or EXPN for security reasons (see below). As 3732 a corollary to the above, implementations that permit this MUST NOT 3733 appear to have verified addresses that are not, in fact, verified. 3734 If a site disables these commands for security reasons, the SMTP 3735 server MUST return a 252 response, rather than a code that could be 3736 confused with successful or unsuccessful verification. 3738 Returning a 250 reply code with the address listed in the VRFY 3739 command after having checked it only for syntax violates this rule. 3740 Of course, an implementation that "supports" VRFY by always returning 3741 550 whether or not the address is valid is equally not in 3742 conformance. 3744 On the public Internet, the contents of mailing lists have become 3745 popular as an address information source for so-called "spammers." 3746 The use of EXPN to "harvest" addresses has increased as list 3747 administrators have installed protections against inappropriate uses 3748 of the lists themselves. However, VRFY and EXPN are still useful for 3749 authenticated users and within an administrative domain. For 3750 example, VRFY and EXPN are useful for performing internal audits of 3751 how email gets routed to check and to make sure no one is 3752 automatically forwarding sensitive mail outside the organization. 3753 Sites implementing SMTP authentication may choose to make VRFY and 3754 EXPN available only to authenticated requestors. Implementations 3755 SHOULD still provide support for EXPN, but sites SHOULD carefully 3756 evaluate the tradeoffs. 3758 Whether disabling VRFY provides any real marginal security depends on 3759 a series of other conditions. In many cases, RCPT commands can be 3760 used to obtain the same information about address validity. On the 3761 other hand, especially in situations where determination of address 3762 validity for RCPT commands is deferred until after the DATA command 3763 is received, RCPT may return no information at all, while VRFY is 3764 expected to make a serious attempt to determine validity before 3765 generating a response code (see discussion above). 3767 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3769 Before a client uses the 251 or 551 reply codes from a RCPT command 3770 to automatically update its future behavior (e.g., updating the 3771 user's address book), it should be certain of the server's 3772 authenticity. If it does not, it may be subject to a man in the 3773 middle attack. 3775 7.5. Information Disclosure in Announcements 3777 There has been an ongoing debate about the tradeoffs between the 3778 debugging advantages of announcing server type and version (and, 3779 sometimes, even server domain name) in the greeting response or in 3780 response to the HELP command and the disadvantages of exposing 3781 information that might be useful in a potential hostile attack. The 3782 utility of the debugging information is beyond doubt. Those who 3783 argue for making it available point out that it is far better to 3784 actually secure an SMTP server rather than hope that trying to 3785 conceal known vulnerabilities by hiding the server's precise identity 3786 will provide more protection. Sites are encouraged to evaluate the 3787 tradeoff with that issue in mind; implementations SHOULD minimally 3788 provide for making type and version information available in some way 3789 to other network hosts. 3791 7.6. Information Disclosure in Trace Fields 3793 In some circumstances, such as when mail originates from within a LAN 3794 whose hosts are not directly on the public Internet, trace 3795 ("Received") header fields produced in conformance with this 3796 specification may disclose host names and similar information that 3797 would not normally be available. This ordinarily does not pose a 3798 problem, but sites with special concerns about name disclosure should 3799 be aware of it. Also, the optional FOR clause should be supplied 3800 with caution or not at all when multiple recipients are involved lest 3801 it inadvertently disclose the identities of "blind copy" recipients 3802 to others. 3804 7.7. Information Disclosure in Message Forwarding 3806 As discussed in Section 3.4, use of the 251 or 551 reply codes to 3807 identify the replacement address associated with a mailbox may 3808 inadvertently disclose sensitive information. Sites that are 3809 concerned about those issues should ensure that they select and 3810 configure servers appropriately. 3812 7.8. Resistance to Attacks 3814 In recent years, there has been an increase of attacks on SMTP 3815 servers, either in conjunction with attempts to discover addresses 3816 for sending unsolicited messages or simply to make the servers 3817 inaccessible to others (i.e., as an application-level denial of 3818 service attack). While the means of doing so are beyond the scope of 3819 this Standard, rational operational behavior requires that servers be 3820 permitted to detect such attacks and take action to defend 3821 themselves. For example, if a server determines that a large number 3822 of RCPT TO commands are being sent, most or all with invalid 3823 addresses, as part of such an attack, it would be reasonable for the 3824 server to close the connection after generating an appropriate number 3825 of 5yz (normally 550) replies. 3827 7.9. Scope of Operation of SMTP Servers 3829 It is a well-established principle that an SMTP server may refuse to 3830 accept mail for any operational or technical reason that makes sense 3831 to the site providing the server. However, cooperation among sites 3832 and installations makes the Internet possible. If sites take 3833 excessive advantage of the right to reject traffic, the ubiquity of 3834 email availability (one of the strengths of the Internet) will be 3835 threatened; considerable care should be taken and balance maintained 3836 if a site decides to be selective about the traffic it will accept 3837 and process. 3839 In recent years, use of the relay function through arbitrary sites 3840 has been used as part of hostile efforts to hide the actual origins 3841 of mail. Some sites have decided to limit the use of the relay 3842 function to known or identifiable sources, and implementations SHOULD 3843 provide the capability to perform this type of filtering. When mail 3844 is rejected for these or other policy reasons, a 550 code SHOULD be 3845 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3847 8. IANA Considerations 3849 IANA maintains three registries in support of this specification, all 3850 of which were created for RFC 2821 or earlier. This document expands 3851 the third one as specified below. The registry references listed are 3852 as of the time of publication; IANA does not guarantee the locations 3853 associated with the URLs. The registries are as follows: 3855 o The first, "Simple Mail Transfer Protocol (SMTP) Service 3856 Extensions" [49], consists of SMTP service extensions with the 3857 associated keywords, and, as needed, parameters and verbs. As 3858 specified in Section 2.2.2, no entry may be made in this registry 3859 that starts in an "X". Entries may be made only for service 3860 extensions (and associated keywords, parameters, or verbs) that 3861 are defined in Standards-Track or Experimental RFCs specifically 3862 approved by the IESG for this purpose. 3864 o The second registry, "Address Literal Tags" [50], consists of 3865 "tags" that identify forms of domain literals other than those for 3866 IPv4 addresses (specified in RFC 821 and in this document). The 3867 initial entry in that registry is for IPv6 addresses (specified in 3868 this document). Additional literal types require standardization 3869 before being used; none are anticipated at this time. 3871 o The third, "Mail Transmission Types" [49], established by RFC 821 3872 and renewed by this specification, is a registry of link and 3873 protocol identifiers to be used with the "via" and "with" 3874 subclauses of the time stamp ("Received:" header field) described 3875 in Section 4.4. Link and protocol identifiers in addition to 3876 those specified in this document may be registered only by 3877 standardization or by way of an RFC-documented, IESG-approved, 3878 Experimental protocol extension. This name space is for 3879 identification and not limited in size: the IESG is encouraged to 3880 approve on the basis of clear documentation and a distinct method 3881 rather than preferences about the properties of the method itself. 3883 An additional subsection has been added to the "VIA link types" 3884 and "WITH protocol types" subsections of this registry to contain 3885 registrations of "Additional-registered-clauses" as described 3886 above. The registry will contain clause names, a description, a 3887 summary of the syntax of the associated String, and a reference. 3888 As new clauses are defined, they may, in principle, specify 3889 creation of their own registries if the Strings consist of 3890 reserved terms or keywords rather than less restricted strings. 3891 As with link and protocol identifiers, additional clauses may be 3892 registered only by standardization or by way of an RFC-documented, 3893 IESG-approved, Experimental protocol extension. The additional 3894 clause name space is for identification and is not limited in 3895 size: the IESG is encouraged to approve on the basis of clear 3896 documentation, actual use or strong signs that the clause will be 3897 used, and a distinct requirement rather than preferences about the 3898 properties of the clause itself. 3900 In addition, if additional trace header fields (i.e., in addition to 3901 Return-path and Received) are ever created, those trace fields MUST 3902 be added to the IANA registry established by BCP 90 (RFC 3864) [8] 3903 for use with RFC 5322 [11]. 3905 9. Acknowledgments 3907 Many people contributed to the development of RFCs 2821 and 5321. 3908 Those documents should be consulted for those acknowledgments. 3910 Neither this document nor RFCs 2821 or 5321 would have been possible 3911 without the many contribution and insights of the late Jon Postel. 3912 Those contributions of course include the original specification of 3913 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3914 appears in this document as do several of Jon's original examples 3915 that have been updated only as needed to reflect other changes in the 3916 specification. 3918 The following filed errata against RFC 5321 that were not rejected at 3919 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3920 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3921 Rodrigo Speller, Alessandro Vesely, and Brett Watson. In addition, 3922 specific suggestions that led to corrections and improvements in this 3923 version were received from Ned Freed, Barry Leiba, Ivar Lumi, Pete 3924 Resnick, and others. 3926 chetti contributed an analysis that clarify the ABNF productions that 3927 implicitly reference other document. 3929 [[CREF27: Most errata and comments after 2019-07-01 have not yet been 3930 captured in this version of the draft. ]] 3932 10. References 3934 10.1. Normative References 3936 [1] Bradner, S., "Key words for use in RFCs to Indicate 3937 Requirement Levels", BCP 14, RFC 2119, 3938 DOI 10.17487/RFC2119, March 1997, 3939 . 3941 [2] American National Standards Institute (formerly United 3942 States of America Standards Institute), "USA Code for 3943 Information Interchange", ANSI X3.4-1968, 1968. 3945 ANSI X3.4-1968 has been replaced by newer versions with 3946 slight modifications, but the 1968 version remains 3947 definitive for the Internet. 3949 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 3950 RFC 821, DOI 10.17487/RFC0821, August 1982, 3951 . 3953 [4] Mockapetris, P., "Domain names - implementation and 3954 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3955 November 1987, . 3957 [5] Braden, R., Ed., "Requirements for Internet Hosts - 3958 Application and Support", STD 3, RFC 1123, 3959 DOI 10.17487/RFC1123, October 1989, 3960 . 3962 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 3963 Extension for Message Size Declaration", STD 10, RFC 1870, 3964 DOI 10.17487/RFC1870, November 1995, 3965 . 3967 [7] Newman, C., "ESMTP and LMTP Transmission Types 3968 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 3969 . 3971 [8] Klyne, G., Nottingham, M., and J. Mogul, "Registration 3972 Procedures for Message Header Fields", BCP 90, RFC 3864, 3973 DOI 10.17487/RFC3864, September 2004, 3974 . 3976 [9] Hinden, R. and S. Deering, "IP Version 6 Addressing 3977 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3978 2006, . 3980 [10] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 3981 Specifications: ABNF", STD 68, RFC 5234, 3982 DOI 10.17487/RFC5234, January 2008, 3983 . 3985 [11] Resnick, P., "Internet Message Format", RFC 5322, 3986 September 2008. 3988 10.2. Informative References 3990 [12] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 3991 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 3992 August 1982, . 3994 [13] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 3995 Reynolds, "Post Office Protocol: Version 2", RFC 937, 3996 DOI 10.17487/RFC0937, February 1985, 3997 . 3999 [14] Postel, J. and J. Reynolds, "File Transfer Protocol", 4000 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4001 . 4003 [15] Partridge, C., "Mail routing and the domain system", 4004 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4005 . 4007 [16] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4008 DOI 10.17487/RFC1047, February 1988, 4009 . 4011 [17] Lambert, M., "PCMAIL: A distributed mail system for 4012 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4013 1988, . 4015 [18] Crispin, M., "Interactive Mail Access Protocol: Version 4016 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4017 . 4019 [19] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4020 DOI 10.17487/RFC1846, September 1995, 4021 . 4023 [20] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4024 "Security Multiparts for MIME: Multipart/Signed and 4025 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4026 October 1995, . 4028 [21] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4029 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4030 DOI 10.17487/RFC1869, November 1995, 4031 . 4033 [22] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4034 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4035 . 4037 [23] De Winter, J., "SMTP Service Extension for Remote Message 4038 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4039 1996, . 4041 [24] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4042 Extensions (MIME) Part One: Format of Internet Message 4043 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4044 . 4046 [25] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4047 Part Three: Message Header Extensions for Non-ASCII Text", 4048 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4049 . 4051 [26] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4052 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4053 DOI 10.17487/RFC2156, January 1998, 4054 . 4056 [27] Elz, R. and R. Bush, "Clarifications to the DNS 4057 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4058 . 4060 [28] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4061 Word Extensions: Character Sets, Languages, and 4062 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4063 1997, . 4065 [29] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4066 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4067 . 4069 [30] Freed, N., "SMTP Service Extension for Command 4070 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4071 September 2000, . 4073 [31] Freed, N., "Behavior of and Requirements for Internet 4074 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4075 . 4077 [32] Vaudreuil, G., "SMTP Service Extensions for Transmission 4078 of Large and Binary MIME Messages", RFC 3030, 4079 DOI 10.17487/RFC3030, December 2000, 4080 . 4082 [33] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4083 Extension for Delivery Status Notifications (DSNs)", 4084 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4085 . 4087 [34] Vaudreuil, G., "Enhanced Mail System Status Codes", 4088 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4089 . 4091 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4092 for Delivery Status Notifications", RFC 3464, 4093 DOI 10.17487/RFC3464, January 2003, 4094 . 4096 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4097 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4098 . 4100 [37] Hansen, T., Ed. and G. Vaudreuil, Ed., "Message 4101 Disposition Notification", RFC 3798, DOI 10.17487/RFC3798, 4102 May 2004, . 4104 [38] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 4105 Extensions (S/MIME) Version 3.1 Message Specification", 4106 RFC 3851, DOI 10.17487/RFC3851, July 2004, 4107 . 4109 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4110 in Mixed IPv4/v6 Environments", RFC 3974, 4111 DOI 10.17487/RFC3974, January 2005, 4112 . 4114 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4115 Resource Identifier (URI): Generic Syntax", STD 66, 4116 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4117 . 4119 [41] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 4120 for Authorizing Use of Domains in E-Mail, Version 1", 4121 RFC 4408, DOI 10.17487/RFC4408, April 2006, 4122 . 4124 [42] Gellens, R. and J. Klensin, "Message Submission for Mail", 4125 RFC 4409, DOI 10.17487/RFC4409, April 2006, 4126 . 4128 [43] Fenton, J., "Analysis of Threats Motivating DomainKeys 4129 Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, 4130 September 2006, . 4132 [44] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 4133 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 4134 Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007, 4135 . 4137 [45] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4138 Thayer, "OpenPGP Message Format", RFC 4880, 4139 DOI 10.17487/RFC4880, November 2007, 4140 . 4142 [46] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4143 Mail System Status Codes", BCP 138, RFC 5248, 4144 DOI 10.17487/RFC5248, June 2008, 4145 . 4147 [47] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4148 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4149 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4150 . 4152 [48] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4153 DOI 10.17487/RFC7504, June 2015, 4154 . 4156 [49] Internet Assigned Number Authority (IANA), "IANA Mail 4157 Parameters", 2007, 4158 . 4160 [50] Internet Assigned Number Authority (IANA), "Address 4161 Literal Tags", 2007, 4162 . 4164 [51] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4165 Record for Domains that Accept No Mail", September 2014, 4166 . 4169 [52] RFC Editor, "RFC Errata - RFC 5321", 2019, 4170 . 4172 Captured 2019-11-19 4174 Appendix A. TCP Transport Service 4176 The TCP connection supports the transmission of 8-bit bytes. The 4177 SMTP data is 7-bit ASCII characters. Each character is transmitted 4178 as an 8-bit byte with the high-order bit cleared to zero. Service 4179 extensions may modify this rule to permit transmission of full 8-bit 4180 data bytes as part of the message body, or, if specifically designed 4181 to do so, in SMTP commands or responses. 4183 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4185 Some systems use an RFC 822 header section (only) in a mail 4186 submission protocol, or otherwise generate SMTP commands from RFC 822 4187 header fields when such a message is handed to an MTA from a UA. 4188 While the MTA-UA protocol is a private matter, not covered by any 4189 Internet Standard, there are problems with this approach. For 4190 example, there have been repeated problems with proper handling of 4191 "bcc" copies and redistribution lists when information that 4192 conceptually belongs to the mail envelope is not separated early in 4193 processing from header field information (and kept separate). 4195 It is recommended that the UA provide its initial ("submission 4196 client") MTA with an envelope separate from the message itself. 4197 However, if the envelope is not supplied, SMTP commands SHOULD be 4198 generated as follows: 4200 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4201 be copied to a RCPT command (generating multiple message copies 4202 if that is required for queuing or delivery). This includes any 4203 addresses listed in a RFC 822 "group". Any BCC header fields 4204 SHOULD then be removed from the header section. Once this 4205 process is completed, the remaining header fields SHOULD be 4206 checked to verify that at least one TO, CC, or BCC header field 4207 remains. If none do, then a BCC header field with no additional 4208 information SHOULD be inserted as specified in [11]. 4210 2. The return address in the MAIL command SHOULD, if possible, be 4211 derived from the system's identity for the submitting (local) 4212 user, and the "From:" header field otherwise. If there is a 4213 system identity available, it SHOULD also be copied to the Sender 4214 header field if it is different from the address in the From 4215 header field. (Any Sender header field that was already there 4216 SHOULD be removed.) Systems may provide a way for submitters to 4217 override the envelope return address, but may want to restrict 4218 its use to privileged users. This will not prevent mail forgery, 4219 but may lessen its incidence; see Section 7.1. 4221 When an MTA is being used in this way, it bears responsibility for 4222 ensuring that the message being transmitted is valid. The mechanisms 4223 for checking that validity, and for handling (or returning) messages 4224 that are not valid at the time of arrival, are part of the MUA-MTA 4225 interface and not covered by this specification. 4227 A submission protocol based on Standard RFC 822 information alone 4228 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4229 system into an SMTP environment. Additional information to construct 4230 an envelope must come from some source in the other environment, 4231 whether supplemental header fields or the foreign system's envelope. 4233 Attempts to gateway messages using only their header "To" and "Cc" 4234 fields have repeatedly caused mail loops and other behavior adverse 4235 to the proper functioning of the Internet mail environment. These 4236 problems have been especially common when the message originates from 4237 an Internet mailing list and is distributed into the foreign 4238 environment using envelope information. When these messages are then 4239 processed by a header-section-only remailer, loops back to the 4240 Internet environment (and the mailing list) are almost inevitable. 4242 Appendix C. Source Routes 4244 Historically, the was a reverse source routing list of 4245 hosts and a source mailbox. The first host in the was 4246 historically the host sending the MAIL command; today, source routes 4247 SHOULD NOT appear in the reverse-path. Similarly, the 4248 may be a source routing lists of hosts and a destination mailbox. 4249 However, in general, the SHOULD contain only a mailbox 4250 and domain name, relying on the domain name system to supply routing 4251 information if required. The use of source routes is deprecated (see 4252 Appendix F.2); while servers MUST be prepared to receive and handle 4253 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT 4254 transmit them and this section is included in the current 4255 specification only to provide context. It has been modified somewhat 4256 from the material in RFC 821 to prevent server actions that might 4257 confuse clients or subsequent servers that do not expect a full 4258 source route implementation. 4260 Historically, for relay purposes, the forward-path may have been a 4261 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4262 THREE MUST be fully-qualified domain names. This form was used to 4263 emphasize the distinction between an address and a route. The 4264 mailbox (here, JOE@THREE) is an absolute address, and the route is 4265 information about how to get there. The two concepts should not be 4266 confused.[[CREF28: [5321bis]JcK 20090123: Tightened this and the next 4267 paragraph to be clear that this doesn't authorize source route use.]] 4268 If source routes are used contrary to requirements and 4269 recommendations elsewhere in this specfiication, RFC 821 and the text 4270 below should be consulted for the mechanisms for constructing and 4271 updating the forward-path. A server that is reached by means of a 4272 source route (e.g., its domain name appears first in the list in the 4273 forward-path) MUST remove its domain name from any forward-paths in 4274 which that domain name appears before forwarding the message and MAY 4275 remove all other source routing information. The reverse-path SHOULD 4276 NOT be updated by servers conforming to this specification. 4278 Notice that the forward-path and reverse-path appear in the SMTP 4279 commands and replies, but not necessarily in the message. That is, 4280 there is no need for these paths and especially this syntax to appear 4281 in the "To:" , "From:", "CC:", etc. fields of the message header 4282 section. Conversely, SMTP servers MUST NOT derive final message 4283 routing information from message header fields. 4285 When the list of hosts is present despite the recommendations and 4286 requirements [[CREF29: [5321bis]JcK 20090123 "and requrements" 4287 added]] above, it is a "reverse" source route and indicates that the 4288 mail was relayed through each host on the list (the first host in the 4289 list was the most recent relay). This list is used as a source route 4290 to return non-delivery notices to the sender. If, contrary to the 4291 recommendations here, a relay host adds itself to the beginning of 4292 the list, it MUST use its name as known in the transport environment 4293 to which it is relaying the mail rather than that of the transport 4294 environment from which the mail came (if they are different). Note 4295 that a situation could easily arise in which some relay hosts add 4296 their names to the reverse source route and others do not, generating 4297 discontinuities in the routing list. This is another reason why 4298 servers needing to return a message SHOULD ignore the source route 4299 entirely and simply use the domain as specified in the Mailbox. 4301 Appendix D. Scenarios 4303 This section presents complete scenarios of several types of SMTP 4304 sessions. In the examples, "C:" indicates what is said by the SMTP 4305 client, and "S:" indicates what is said by the SMTP server. 4307 D.1. A Typical SMTP Transaction Scenario 4309 This SMTP example shows mail sent by Smith at host bar.com, and to 4310 Jones, Green, and Brown at host foo.com. Here we assume that host 4311 bar.com contacts host foo.com directly. The mail is accepted for 4312 Jones and Brown. Green does not have a mailbox at host foo.com. 4314 S: 220 foo.com Simple Mail Transfer Service Ready 4315 C: EHLO bar.com 4316 S: 250-foo.com greets bar.com 4317 S: 250-8BITMIME 4318 S: 250-SIZE 4319 S: 250-DSN 4320 S: 250 HELP 4321 C: MAIL FROM: 4322 S: 250 OK 4323 C: RCPT TO: 4324 S: 250 OK 4325 C: RCPT TO: 4326 S: 550 No such user here 4327 C: RCPT TO: 4328 S: 250 OK 4329 C: DATA 4330 S: 354 Start mail input; end with . 4331 C: Blah blah blah... 4332 C: ...etc. etc. etc. 4333 C: . 4334 S: 250 OK 4335 C: QUIT 4336 S: 221 foo.com Service closing transmission channel 4338 D.2. Aborted SMTP Transaction Scenario 4340 S: 220 foo.com Simple Mail Transfer Service Ready 4341 C: EHLO bar.com 4342 S: 250-foo.com greets bar.com 4343 S: 250-8BITMIME 4344 S: 250-SIZE 4345 S: 250-DSN 4346 S: 250 HELP 4347 C: MAIL FROM: 4348 S: 250 OK 4349 C: RCPT TO: 4350 S: 250 OK 4351 C: RCPT TO: 4352 S: 550 No such user here 4353 C: RSET 4354 S: 250 OK 4355 C: QUIT 4356 S: 221 foo.com Service closing transmission channel 4358 D.3. Relayed Mail Scenario 4360 Step 1 -- Source Host to Relay Host 4362 The source host performs a DNS lookup on XYZ.COM (the destination 4363 address) and finds DNS MX records specifying xyz.com as the best 4364 preference and foo.com as a lower preference. It attempts to open a 4365 connection to xyz.com and fails. It then opens a connection to 4366 foo.com, with the following dialogue: 4368 S: 220 foo.com Simple Mail Transfer Service Ready 4369 C: EHLO bar.com 4370 S: 250-foo.com greets bar.com 4371 S: 250-8BITMIME 4372 S: 250-SIZE 4373 S: 250-DSN 4374 S: 250 HELP 4375 C: MAIL FROM: 4376 S: 250 OK 4377 C: RCPT TO: 4378 S: 250 OK 4379 C: DATA 4380 S: 354 Start mail input; end with . 4381 C: Date: Thu, 21 May 1998 05:33:29 -0700 4382 C: From: John Q. Public 4383 C: Subject: The Next Meeting of the Board 4384 C: To: Jones@xyz.com 4385 C: 4386 C: Bill: 4387 C: The next meeting of the board of directors will be 4388 C: on Tuesday. 4389 C: John. 4390 C: . 4391 S: 250 OK 4392 C: QUIT 4393 S: 221 foo.com Service closing transmission channel 4395 Step 2 -- Relay Host to Destination Host 4397 foo.com, having received the message, now does a DNS lookup on 4398 xyz.com. It finds the same set of MX records, but cannot use the one 4399 that points to itself (or to any other host as a worse preference). 4400 It tries to open a connection to xyz.com itself and succeeds. Then 4401 we have: 4403 S: 220 xyz.com Simple Mail Transfer Service Ready 4404 C: EHLO foo.com 4405 S: 250 xyz.com is on the air 4406 C: MAIL FROM: 4407 S: 250 OK 4408 C: RCPT TO: 4409 S: 250 OK 4410 C: DATA 4411 S: 354 Start mail input; end with . 4412 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4413 C: 05:33:29 -0700 4414 C: Date: Thu, 21 May 1998 05:33:29 -0700 4415 C: From: John Q. Public 4416 C: Subject: The Next Meeting of the Board 4417 C: To: Jones@xyz.com 4418 C: 4419 C: Bill: 4420 C: The next meeting of the board of directors will be 4421 C: on Tuesday. 4422 C: John. 4423 C: . 4424 S: 250 OK 4425 C: QUIT 4426 S: 221 xyz.com Service closing transmission channel 4428 D.4. Verifying and Sending Scenario 4430 S: 220 foo.com Simple Mail Transfer Service Ready 4431 C: EHLO bar.com 4432 S: 250-foo.com greets bar.com 4433 S: 250-8BITMIME 4434 S: 250-SIZE 4435 S: 250-DSN 4436 S: 250-VRFY 4437 S: 250 HELP 4438 C: VRFY Crispin 4439 S: 250 Mark Crispin 4440 C: MAIL FROM: 4441 S: 250 OK 4442 C: RCPT TO: 4443 S: 250 OK 4444 C: DATA 4445 S: 354 Start mail input; end with . 4446 C: Blah blah blah... 4447 C: ...etc. etc. etc. 4448 C: . 4449 S: 250 OK 4450 C: QUIT 4451 S: 221 foo.com Service closing transmission channel 4453 Appendix E. Other Gateway Issues 4455 In general, gateways between the Internet and other mail systems 4456 SHOULD attempt to preserve any layering semantics across the 4457 boundaries between the two mail systems involved. Gateway- 4458 translation approaches that attempt to take shortcuts by mapping 4459 (such as mapping envelope information from one system to the message 4460 header section or body of another) have generally proven to be 4461 inadequate in important ways. Systems translating between 4462 environments that do not support both envelopes and a header section 4463 and Internet mail must be written with the understanding that some 4464 information loss is almost inevitable. 4466 Appendix F. Deprecated Features of RFC 821 4468 A few features of RFC 821 have proven to be problematic and SHOULD 4469 NOT be used in Internet mail. Some of these features were deprecated 4470 in RFC 2821 in 2001; source routing and two-digit years in dates were 4471 deprecated by RFC 1123 in 1989. Of the domain literal forms, RFC 4472 1123 required support only for the dotted decimal form. With the 4473 possible exception of old, hardware-embedded, applications, there is 4474 no longer any excuse for these features to appear on the contemporary 4475 Internet. [[CREF30: [5321bis] (2821ter) 2821bis Last Call Comment]] 4477 F.1. TURN 4479 This command, described in RFC 821, raises important security issues 4480 since, in the absence of strong authentication of the host requesting 4481 that the client and server switch roles, it can easily be used to 4482 divert mail from its correct destination. Its use is deprecated; 4483 SMTP systems SHOULD NOT use it unless the server can authenticate the 4484 client. 4486 F.2. Source Routing 4488 RFC 821 utilized the concept of explicit source routing to get mail 4489 from one host to another via a series of relays. The requirement to 4490 utilize source routes in regular mail traffic was eliminated by the 4491 introduction of the domain name system "MX" record and the last 4492 significant justification for them was eliminated by the 4493 introduction, in RFC 1123, of a clear requirement that addresses 4494 following an "@" must all be fully-qualified domain names. 4495 Consequently, the only remaining justifications for the use of source 4496 routes are support for very old SMTP clients or MUAs and in mail 4497 system debugging. They can, however, still be useful in the latter 4498 circumstance and for routing mail around serious, but temporary, 4499 problems such as problems with the relevant DNS records. 4501 SMTP servers MUST continue to accept source route syntax as specified 4502 in the main body of this document and in RFC 1123. They MAY, if 4503 necessary, ignore the routes and utilize only the target domain in 4504 the address. If they do utilize the source route, the message MUST 4505 be sent to the first domain shown in the address. In particular, a 4506 server MUST NOT guess at shortcuts within the source route. 4508 Clients SHOULD NOT utilize explicit source routing except under 4509 unusual circumstances, such as debugging or potentially relaying 4510 around firewall or mail system configuration errors. 4512 F.3. HELO 4514 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4515 than HELO when the server will accept the former. Servers MUST 4516 continue to accept and process HELO in order to support older 4517 clients. 4519 F.4. #-literals 4521 RFC 821 provided for specifying an Internet address as a decimal 4522 integer host number prefixed by a pound sign, "#". In practice, that 4523 form has been obsolete since the introduction of TCP/IP. It is 4524 deprecated and MUST NOT be used. 4526 F.5. Dates and Years 4528 When dates are inserted into messages by SMTP clients or servers 4529 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4530 digit years are deprecated; three-digit years were never permitted in 4531 the Internet mail system. 4533 F.6. Sending versus Mailing 4535 In addition to specifying a mechanism for delivering messages to 4536 user's mailboxes, RFC 821 provided additional, optional, commands to 4537 deliver messages directly to the user's terminal screen. These 4538 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4539 workstation technology and the introduction of other protocols may 4540 have rendered them obsolete even where they are implemented. 4541 [[5321bis Editor's Note: does this need a stronger reference to 821, 4542 2821, and/or 5321?]] 4544 Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers 4545 MAY implement them. If they are implemented by servers, the 4546 implementation model specified in RFC 821 MUST be used and the 4547 command names MUST be published in the response to the EHLO command. 4549 Appendix G. Other Outstanding Issues 4551 [[RFC Editor: Please remove this section before publication.]] 4553 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4554 that led to a broad discussion of ways in which existing practice had 4555 diverged from the specifications and recommendations of RFC 5321 in 4556 the more than eleven years since it was published (some of those 4557 issues probably affect the boundary between RFC 5321 and 5322 and 4558 hence the latter as well). In most cases, those divergences call for 4559 revision of the Technical Specification to match the practice, 4560 clarification of the specification text in other ways, or a more 4561 comprehensive explanation of why the practices recommended by the 4562 specification should really be followed. 4564 Those discussions raised two other issues, which were that 4566 o The publication of the Submission Server specification of RFC 6409 4567 in November 2011 may not have been fully reflected in RFC 5321 4568 (despite the even earlier publication of RFC 4409) and 4570 o There may be inconsistencies between the July 2009 Internet Mail 4571 Architecture description of RFC 5598 and the model described in 4572 RFC 5321. The issue called out in Appendix G.3 below may be an 4573 example of one of those inconsistencies. 4575 Those discrepancies should be identified and discussed and decisions 4576 made to fix them (and where) or to ignore them and let them continue. 4578 There has also been discussion on the mailing list, perhaps amounting 4579 to very rough consensus, that any revision of RFC 5321 and/or 5322 4580 should be accompanied by a separate Applicability Statement document 4581 that would make recommendations about applicability or best practices 4582 in particular areas rather than trying to get everything into the two 4583 technical specifications. This appendix does not attempt to identify 4584 which issues should get which treatment. 4586 Until and unless there is a WG with appropriate leadership and 4587 tracking mechanisms, this appendix will act as a temporary record of 4588 issues that should be discussed and decided upon before a revised 4589 SMTP specification (or a related Applicability Statement) is 4590 published, issues that have not been reflected in errata (see 4591 Appendix H.1 below for those covered by errata). 4593 G.1. IP Address Literals 4595 The specification is unclear about whether IP address literals, 4596 particularly IP address literals used as arguments to the EHLO 4597 command, are required to be accepted or whether they are allowed to 4598 be rejected as part of the general "operational necessity" exception. 4599 Some have suggested that rejection of them is so common as an anti- 4600 spam measure that the use of such literals should be deprecated 4601 entirely in the specification, others that the are still useful and 4602 used and/or that, whatever is said about IP address literals within 4603 an SMTP session (e.g., in MAIL or RCPT commands), they should 4604 continue to be allowed (and required) in EHLO. 4606 G.2. Repeated Use of EHLO 4608 While the specification says that an SMTP client's sending EHLO again 4609 after it has been issued (starting an SMTP session and treats it as 4610 if RSET had been sent (closing the session) followed by EHLO, there 4611 are apparently applications, at least some of them involving setting 4612 up of secure connections, in which the second EHLO is required and 4613 does not imply RSET. Does the specification need to be adjusted to 4614 reflect or call out those cases? 4616 G.3. Meaning of "MTA" and Related Terminology 4618 A terminology issue has come up about what the term "MTA" actually 4619 refers to, a question that became at least slightly more complicated 4620 when we formalized RFC 6409 Submission Servers. Does the document 4621 need to be adjusted to be more clear about this topic? Note that the 4622 answer may interact with the question asked in Section 2 above. 4623 Possibly along the same lines, RFC 2821 changed the RFC 821 4624 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4625 and "SMTP server" respectively. As things have evolved, it is 4626 possible that newer terminology is a source of confusion and that the 4627 terminology should be changed back, something that also needs 4628 discussion. 4630 G.4. Originator, or Originating System, Authentication 4632 Should RFC 5321bis address authentication and related issues or 4633 should Section 3.9 or other text be reshaped (in addition to or 4634 instead of the comment on that section) to lay a better foundation 4635 for such work, either in the context of mailing lists or more 4636 generally? 4638 G.5. Remove or deprecate the work-around from code 552 to 452 4640 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4641 and/or be inconsistent with current practice. Should it be removed 4642 and/or explicitly deprecated? 4644 G.6. Clarify where the protocol stands with respect to submission and 4645 TLS issues 4647 1. submission on port 587 4649 2. submission on port 465 4651 3. TLS relay on a port different from 25 (whenever) 4653 G.7. Probably-substantive Discussion Topics Identified in Other Ways 4655 The following issues were identified as a group in the opening Note 4656 but called out specifically only in embedded CREF comments in earlier 4657 (-00 and -01) versions of this draft. 4659 G.7.1. Issues with 521, 554, and 556 codes 4661 See new Section 4.2.4.2. More text may be needed, there or 4662 elsewhere, about choices of codes in response to initial opening and 4663 to EHLO, especially to deal with selective policy rejections. 4665 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 4667 CREF comment in Section 2 and also CREF comment in Section 2.3.10 4669 G.7.3. Resolvable FQDNs and private domain names 4671 Multiple CREF comments in Section 2.3.5 4673 G.7.4. Possible clarification about mail transactions and transaction 4674 state 4676 CREF comment in Section 3.3 and also reference in Section 4.1.4 4678 G.7.5. Issues with mailing lists, aliases, and forwarding 4680 CREF comment in Section 3.9. May also want to note forwarding as an 4681 email address portability issue. 4683 G.7.6. Requirements for domain name and/or IP address in EHLO 4685 CREF comment in Section 4.1.4 4687 G.7.7. Does the 'first digit only' and/or non-listed reply code text 4688 need clarification? 4690 CREF comments in Section 4.2 and Section 4.3.1 4692 G.7.8. Size limits 4694 CREF comment in Section 4.5.3 4696 G.7.9. Discussion of 'blind' copies and RCPT 4698 CREF comment in Section 7.2. May alto need to discussion whether 4699 that terminology is politically incorrect and suggest a replacement. 4701 G.7.10. Further clarifications needed to source routes? 4703 CREF comment in Appendix C 4705 G.7.11. Should 1yz Be Revisited? 4707 RFC 5421 depreciated the "positive preliminary reply" response code 4708 category with first digit "1", so that the first digit of valid SMTP 4709 response codes must be 2, 3, 4, or 5. It has been suggested (see 4710 mail from Hector Santos with Subject "SMTP Reply code 1yz Positive 4711 Preliminary reply", March 5, 2020 12:56 -0500, on the SMTP list) that 4712 these codes should be reinstated to deal with some situations that 4713 became more plausible after 5321 was published. Do we need to take 4714 this back up? 4716 G.7.12. Review Timeout Specifications 4718 RFC 5321 (and its predecessors going back to 821) specify minimum 4719 periods for client and server to wait before timing out. Are those 4720 intervals still appropriate in a world of faster processors and 4721 faster networks? Should they be updated and revised? Or should more 4722 qualifying language be added? 4724 G.8. Enhanced Reply Codes and DSNs 4726 Enhanced Mail System Status Codes [34] were added to SMTP before RFC 4727 5321 was published and are now, together with a corresponding 4728 registry [46], widely deployed and in extensive use in the network. 4729 Similar, the structure and extensions options for Delivery Status 4730 Notifications [35] is implemented, deployed, and in wide use. Is it 4731 time to fold all or part of those mature specifications into the SMTP 4732 spec or at least to mention and normatively reference them? And, as 4733 an aside do those specs need work or, if they are kept separate, is 4734 it time to move them to Internet Standard? 4736 G.9. Revisiting Quoted Strings 4738 Recent discussions both in and out of the IETF have highlighted 4739 instances of non-compliance with the specification of a Local-part 4740 consisting of a Quoted-string, whether any content of QcontentSMTP 4741 that actually requires special treatment consists of qtextSMTP, 4742 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4743 above) ends with a few paragraphs of warnings (essentially a partial 4744 applicability statement), the first of which cautions against 4745 cleverness with either Quoted-string or case sensitivity as a threat 4746 to interoperability. 4748 The Quoted-string portion of that discussion has apparently been 4749 widely not read or ignored. Do we need to do something else? If we 4750 do an Applicability Statement, would it be useful to either reference 4751 the discussion in this document from there or to move the discussion 4752 there and reference it (normatively?) from here? 4754 G.10. Internationalization 4756 RFC 5321 came long before work on internationalization of email 4757 addresses and headers (other than by use of encoded words in MINE) 4758 and specifically before the work of the EAI WG leading to the 4759 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4760 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4761 define mailboxes ...") is an extremely strong prohibition against the 4762 use of non-ASCII characters. Would it be appropriate to add 4763 something like "in the absence of relevant extensions" there? Also, 4764 given [mis]behavior seen in the wild, does that paragraph (or an A/S) 4765 need an explicit caution about SMTP servers or clients assuming they 4766 can apply the popular web convention of using %NN sequences as a way 4767 to encode non-ASCII characters ( in RFC 3986) and 4768 assuming some later system will interpret it as they expect? Would 4769 it be appropriate to add an Internationalization Considerations 4770 section to the body of this document if only for the purpose of 4771 pointing people elsewhere? 4773 G.11. SMTP Clients, Servers, Senders, and Receivers 4775 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4776 2821 (and hence in 5321), we switched that to "client" and "server" 4777 (See the discussion in Section 1.2). In part because a relay is a 4778 server and then a client (in some recent practice, even interleaving 4779 the two functions by opening the connection to the next host in line 4780 and sending commands before the incoming transaction is complete), 4781 RFC 5321 continues to use the original terminology in some places. 4782 Should we revisit that usage, possibly even returning to consistent 4783 use of the original terminology? 4785 Appendix H. Change log for RFC 5321bis 4787 [[RFC Editor: Please remove this section before publication.]] 4789 H.1. RFC 5321 Errata Summary 4791 This document addresses the following errata filed against RFC 5321 4792 since its publication in October 2008 [52] [[CREF31: [[Note in Draft: 4793 Items with comments below have not yet been resolved.]]]] 4795 1683 ABNF error. Section 4.4 4797 4198 Description error. Section 4.2 4799 2578 Syntax description error. Section 4.1.2 4801 1543 Wrong code in description Section 3.8 4803 4315 ABNF - IPv6 Section 4.1.3. [[CREF32: [5321bis]The IPv6 syntax 4804 has been adjusted since 5321 was published. See the rewritten 4805 form and the comment in the section cited in the previous 4806 sentence. The editor awaits instructions. See https://www.rfc- 4807 editor.org/errata/eid4315]] 4809 5414 ABNF for Quoted-string Section 4.1.2 4811 1851 Location of text on unexpected close Section 4.1.1.5. 4812 [[CREF33: [5321bis]Matter of taste, editor seeks advice.]] 4814 3447 Use of normative language (e.g., more "MUST"s), possible 4815 confusion in some sections Section 4.4. [[CREF34: [5321bis]As 4816 Barry notes in his verifier comments on the erratum (see 4817 https://www.rfc-editor.org/errata/eid3447), the comments and 4818 suggestions here raise a number of interesting (and difficult) 4819 issues. One of the issues is that the core of RFCs 5321 (and 4820 2821) is text carried over from Jon Postel's RFC 821, a document 4821 that was not only written in a different style than the IETF uses 4822 today but that was written at a time when no one had dreamt of RFC 4823 2119 or even the IETF itself. It appears to me that trying to 4824 patch that style might easily result in a document that is harder 4825 to read as well as being error prone. If we want to get the 4826 document entirely into contemporary style, we really should bite 4827 the bullet and do a complete rewrite. To respond to a different 4828 point in Barry's discussion, I think an explicit statement that 4829 5321/5322 and their predecessors differ in places and why would be 4830 helpful. Text, and suggestions about where to put it, are 4831 solicited. A list of differences might be a good idea too, but 4832 getting it right might be more work than there is available energy 4833 to do correctly. ]] 4835 5711 Missing leading spaces in example Appendix D.3. [[CREF35: 4836 [5321bis]Well, this is interesting because the XML is correct and 4837 the spaces are there, embedded in artwork. So either the XML2RFC 4838 processor at the time took those leading spaces out or the RFC 4839 Editor improved on the document and the change was not caught in 4840 AUTH48, perhaps because rfcdiff ignores white space. We just need 4841 to watch for future iterations. ]] 4843 [[CREF36: [5321bis]Note that rejected errata have _not_ been reviewed 4844 to see if they contain anything useful that should be discussed again 4845 with the possibility of rethinking and changing text. Volunteers 4846 sought.]] 4848 H.2. Changes from RFC 5321 (published October 2008) to the initial 4849 (-00) version of this draft 4851 o Acknowledgments section (Section 9) trimmed back for new document. 4853 o Introductory paragraph to Appendix F extended to make it clear 4854 that these features were deprecated a long time ago and really 4855 should not be in use any more. 4857 o Adjusted some language to clarify that source routes really, 4858 really, should not be used or depended upon. 4860 o IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 4861 a note added. 4863 o Production index added as a first step in tying all productions to 4864 their sources. As part of the effort to make the document more 4865 easily navigable, table of contents entries have been created for 4866 the individual command descriptions. 4868 o Clarified the relationship between the SMTP "letters, digits, and 4869 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 4871 o Revised the reply code sections to add new 521 and 556 codes, 4872 clarify relationships, and be explicit about the requirement for 4873 clients to rely on first digits rather than the sequences in 4874 Section 4.3.2. 4876 o In conjunction with the above, explicitly obsolete RFCs 1846 and 4877 7504. 4879 o Incorporated a correction reflecting Errata ID 2578. 4881 o Some small editorial changes made to eliminate redundant 4882 statements that were very close together. Other, equally small, 4883 editorial changes have been made to improve grammar or clarity. 4885 o A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 4886 Draft" have been added for the group to resolve. Other questions, 4887 especially those in the errata summary, are simply included in 4888 narrative comments in CREFs. 4890 o Checked and rationalized "response" (to a command) and "reply 4891 code" terminology. One can talk about a "999 response" but only a 4892 "999 reply code". There is no such thing as a "response code". 4894 o Added note about length limit on mailbox names ("email 4895 addresses"). 4897 o Added an "errata summary" subsection to this change log/ 4898 comparison to 5321 in this Appendix. The entire Appendix will, of 4899 course, disappear at the time of RFC publication unless someone 4900 wants to make a strong case for retaining it. 4902 o Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 4903 readers below the Abstract. 4905 o Temporarily added a "Note on Reading This Working Draft" after the 4906 Abstract. 4908 H.3. Changes Among Versions of Rfc5321bis 4910 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 4911 -01 4913 Substantively, these two versions differ only by suppression of the 4914 CREF and other discussion associated with the evolution from RFC 2821 4915 to RFC 5321. That change includes an update to the document's Note 4916 to Readers, the date, the file name, and the addition of this change 4917 log subsection. 4919 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 4921 o Minor clarifications to improve text, e.g., addition of NOOP to 4922 the list of non-mail transaction examples in Section 4.1.4. 4924 o Added topics exposed in the ietf-smtp list and the IETF list 4925 "dogfood" discussion during December 2019 and an index listing of 4926 substantive issues identified only in CREFs in the prior draft as 4927 a new Appendix G.. 4929 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 4931 o Added more text to Appendix G.7.1 to specifically call out the 4932 session-opening policy issues surrounding these codes. 4934 o Added discussion of "1yz" reinstatement in Appendix G.7.11. 4936 o Added discussion of timeouts in Appendix G.7.12. 4938 o Added subsection on Enhanced Status Codes and DSNs to the 4939 outstanding issues list Appendix G.8. 4941 o Replaced reference to RFC 1652 (8BITMIME) with the Internet 4942 Standard version, RFC 6152. 4944 o With help from cketti, clarified the ABNF productions whose 4945 terminals appear in other documents. 4947 o Added discussions of Quoted-string, Internationalization, and 4948 client-server versus sender-receiver terminology to Appendix G. 4950 o Added note to the Abstract. 4952 Index 4954 A 4955 Argument Syntax 4956 A-d-l 42 4957 Additional-Registered-Clauses 63 4958 address-literal 43 4959 Addtl-Link 63 4960 Addtl-Protocol 63 4961 ALPHA 42 4962 Argument 42 4963 At-domain 42 4964 atext 42 4965 Atom 43 4966 By-domain 62 4967 CFWS 42 4968 CRLF 42 4969 dcontent 45 4970 DIGIT 42 4971 Domain 43 4972 Dot-string 43 4973 esmtp-keyword 42 4974 esmtp-param 42 4975 esmtp-value 42 4976 Extended-Domain 62 4977 For 63 4978 Forward-Path 42 4979 From-domain 62 4980 FWS 42 4981 General-address-literal 45 4982 Greeting 48 4983 h16 45 4984 HEXDIG 42 4985 ID 63 4986 IPv4-address-literal 45 4987 IPv6-addr 45 4988 IPv6-address-literal 45 4989 Keyword 42 4990 Ldh-str 43 4991 Let-dig 43 4992 Link 63 4993 Local-part 43 4994 ls32 45 4995 Mail-parameters 42 4996 Mailbox 43 4997 Opt-info 62 4998 Path 42 4999 Protocol 63 5000 QcontentSMTP 43 5001 qtextSMTP 43 5002 quoted-pairSMTP 43 5003 Quoted-string 43 5004 Rcpt-parameters 42 5005 Reply-code 48 5006 Reply-line 48 5007 Return-path-line 62 5008 Reverse-Path 42 5009 Snum 45 5010 SP 42 5011 Stamp 62 5012 Standardized-tag 45 5013 String 43 5014 sub-domain 43 5015 TCP-info 62 5016 textstring 48 5017 Time-stamp-line 62 5018 Via 62 5019 With 62 5021 C 5022 Command Syntax 5023 data 39 5024 expn 40 5025 help 40 5026 mail 36 5027 noop 41 5028 quit 41 5029 rcpt 38 5030 rset 39 5031 vrfy 40 5033 Author's Address 5035 John C. Klensin 5036 1770 Massachusetts Ave, Suite 322 5037 Cambridge, MA 02140 5038 USA 5040 EMail: john-ietf@jck.com