Simple Mail Transfer Protocol
RFC 5321

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• 11

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 . . . . . . . . . . . . . . . . . . . . . . . . . 31