SMTP Service Extensions (ESMTP)

Overview

SMTP Service Extensions, written as "ESMTP" and characterized in the literature as "Extended SMTP" (and sometimes "Enhanced SMTP") is a framework for extending SMTP. Extensions are expected to be registered with IANA and negotiated between the client and server.

Whereas SMTP mandates that a session begins with the "HELO" command, ESMTP requires the "EHLO" (extended hello) command. Note that nowadays even "standard" SMTP uses EHLO as its initial greeting, and only falls back to HELO, so in a sense every SMTP server has effectively become an ESMTP server.

The ESMTP specification itself no longer actually exists independently of SMTP. We mention it here mostly for completion, historical purposes, and as a reference to a concept that is still very much in use, albeit now as a concept internal to SMTP.

Specifications

RFC1425

RFC1425 ("SMTP Service Extensions") is the originally published ESMTP specification. It was obsoleted by RFC1651.

RFC1651

RFC1651 ("SMTP Service Extensions") obsoletes RFC1425 and was itself obsoleted by RFC1869.

RFC1869

RFC1869 ("SMTP Service Extensions") obsoletes RFC1651, and is essentially the final specification in which ESMTP was defined separately from SMTP. When this spec was obsoleted by RFC2821, it was incorporated into the SMTP specification.

  • SMTP, because ESMTP defines an extension framework for SMTP

  • LMTP, as LMTP is defined by the ESMTP framework