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.