Simple Mail Transfer Protocol

Document Type Active Internet-Draft (emailcore WG)
Author John Klensin 
Last updated 2021-02-20
Replaces draft-klensin-rfc5321bis
Stream Internet Engineering Task Force (IETF)
Intended RFC status Internet Standard
Formats plain text pdf htmlized (tools) htmlized bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Yes
Telechat date
Responsible AD (None)
Send notices to (None)
EMAILCORE                                                     J. Klensin
Internet-Draft                                         February 21, 2021
Obsoletes: 5321, 1846, 7504 (if
Intended status: Standards Track
Expires: August 25, 2021

                     Simple Mail Transfer Protocol


   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.  This document replaces the earlier version with the
   same title in RFC 5321.
   [[CREF1: Note in Draft: Except for the last sentence, the above is
   unchanged from 5321 and may need adjusting in the light of RFC 6409
   (Message Submission) as an Internet Standard.]]

Note on Reading This Working Draft

   This working draft is extensively annotated with information about
   changes made over the decade since RFC 5321 appeared, especially when
   those changes might be controversial or should get careful review.
   Anything marked in CREF comments with "[5321bis]" is current.  In
   general, unless those are marked with "[[Note in Draft", in the
   contents of an "Editor's note", or are in the "Errata Summary"
   appendix (Appendix H.1, they are just notes on changes that have
   already been made and where those changes originated.  Comments
   identified as "2821ter" arose after the Last Call on what became
   RFC5321, sometimes before AUTH48 on that document or a bit later.
   Those, of course, should still be reviewed.  Surviving comments about
   rfc5321bis-00 followed by a letter indicate intermediate working
   versions of this draft and can be ignored unless the origin of
   changes is important.  As one can tell from the dates (when they are
   given), this document has been periodically updated over a very long
   period of time.

Klensin                  Expires August 25, 2021                [Page 1]
Internet-Draft                    SMTP                     February 2021

   As people review or try to use this document, it may be worth paying
   special attention to the historical discussion in Section 1.2.  The
   decision to merge documents rather than do a complete rewrite was
   motivated by weighing the risks of inadvertently introducing changes
   against greater readability and deciding to preserve close
   approximations to original text and document structures in most
   cases.  One result is that information may not be be organized as the
   reader might expect.  An index is provided to assist in the quest for

   This evolving draft should be discussed on the

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 25, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Klensin                  Expires August 25, 2021                [Page 2]
Internet-Draft                    SMTP                     February 2021

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1.  Transport of Electronic Mail  . . . . . . . . . . . . . .   6
Show full document text