Simple Mail Transfer Protocol
RFC 5321
Document | Type |
RFC - Draft Standard
(October 2008; Errata)
Updated by RFC 7504
Obsoletes RFC 2821
Updates RFC 1123
Was draft-klensin-rfc2821bis (individual in app area)
|
|
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Author | John Klensin | ||
Last updated | 2020-01-21 | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized with errata bibtex | ||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 5321 (Draft Standard) | |
Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Lisa Dusseault | ||
Send notices to | tony@att.com, chris.newman@sun.com |
Network Working Group J. Klensin Request for Comments: 5321 October 2008 Obsoletes: 2821 Updates: 1123 Category: Standards Track Simple Mail Transfer Protocol Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract This document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a "mail submission" protocol for "split-UA" (User Agent) mail reading systems and mobile environments. Klensin Standards Track [Page 1] RFC 5321 SMTP October 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . . 5 1.2. History and Context for This Document . . . . . . . . . . 5 1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 7 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 9 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2. Definition and Registration of Extensions . . . . . . 10 2.2.3. Special Issues with Extensions . . . . . . . . . . . . 11 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . . 11 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . . 11 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 12 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . . 12 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . . 13 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . . 14 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . . 14 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 15 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . . 15 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 15 2.4. General Syntax Principles and Transaction Model . . . . . 16 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . . 17 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 18 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 18 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 19 3.4. Forwarding for Address Correction or Updating . . . . . . 21 3.5. Commands for Debugging Addresses . . . . . . . . . . . . . 22 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 22 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . . 24 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . . 25 3.5.4. Semantics and Applications of EXPN . . . . . . . . . . 26 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 26 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . . 26 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . . 26 3.6.3. Message Submission Servers as Relays . . . . . . . . . 27 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 28 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 28 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . . 29 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 29 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 29 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 30 3.8. Terminating Sessions and Connections . . . . . . . . . . . 30 3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 31 3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 31 Klensin Standards Track [Page 2] RFC 5321 SMTP October 2008 3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . . 31Show full document text