D. J. Bernstein
Internet mail

SMTP: Simple Mail Transfer Protocol

Requests, verbs, parameters, responses, and codes

How the client transfers mail
Encoded messages
Encoded addresses
The MAIL, RCPT, and DATA verbs
The HELO, RSET, and NOOP verbs
The QUIT verb

The EHLO verb and SMTP extensions
The PIPELINING extension
The 8BITMIME extension
The SIZE extension

The HELP verb
The VRFY and EXPN verbs
The SEND, SAML, and SOML verbs
The TURN and ETRN verbs

These pages are designed as a reference for the Simple Mail Transfer Protocol (SMTP). They cover today's SMTP implementations and the IETF SMTP specifications, including some SMTP extensions.

There are several IETF SMTP specifications:

  1. RFC 821, published in 1982.
  2. RFC 1123, published in 1989. A few corrections to RFC 821.
  3. RFC 1425, published in 1983. Introduced the EHLO verb.
  4. RFC 1651, published in 1994. Replaced RFC 1425.
  5. RFC 1869, published in 1995. Replaced RFC 1651.
  6. RFC 2821, published in 2001, code-named Klensin. Replaced 821/1123/1869.