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RFC 5321: Simple Mail Transfer Protocol

Still current, but amended. Parts of this document are changed or extended by RFC 7504. Read both.

In plain English — editorial summary, not part of the RFC

RFC 5321 defines SMTP, the protocol mail servers use to hand messages to each other. It covers the command dialogue (EHLO, MAIL FROM, RCPT TO, DATA), the envelope addresses that actually determine delivery, reply codes, and relaying rules. The envelope sender it defines is separate from the From: header a user sees — a distinction that matters constantly in deliverability, SPF and bounce handling.

Original abstract from the RFC

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. [STANDARDS-TRACK]

Document record

Document ID
RFC5321
Published
October 2008
Authors
J. Klensin
Status
DRAFT STANDARD
Stream
IETF
Area
—
Pages
95
Also known as
—
Obsoletes:
RFC 2821
Updates:
RFC 1123
Updated by:
RFC 7504

Topics

Standards lineage

This document is one revision in a chain of 10 RFCs, each formally replacing the one before it.

  1. RFC 772 (1980)
  2. RFC 780 (1981)
  3. RFC 788 (1981)
  4. RFC 821 (1982)
  5. RFC 974 (1986)
  6. RFC 1425 (1993)
  7. RFC 1651 (1994)
  8. RFC 1869 (1995)
  9. RFC 2821 (2001)
  10. RFC 5321 (2008)

Read the full history of Simple Mail Transfer Protocol →

Referenced by

One later RFC formally updates or obsoletes part of this document.

Related documents

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About this page

The document record above — title, authors, date, status, stream, area, relationships, DOI and errata — is imported verbatim from the public RFC Editor index. The “in plain English” section is editorial: written by The metasystema editorial team, not part of the RFC. Where the two differ, the RFC text governs.

Last checked against the RFC Editor index on . RFCs are never revised after publication; changes are issued as new documents.

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canonical URL: /rfc/5321-simple-mail-transfer-protocol