Editing Simple Mail Transfer Protocol (section)

===Extension discovery mechanism===
Clients learn a server's supported options by using the <code>EHLO</code> greeting, as exemplified below, instead of the original <code>HELO</code>. Clients fall back to <code>HELO</code> only if the server does not support <code>EHLO</code> greeting.<ref>{{cite IETF |title=SMTP Service Extensions |rfc=1869 |author1=John Klensin |author2=Ned Freed |author3=Marshall T. Rose |author4=Einar A. Stefferud |author5=Dave Crocker |date=November 1995 |publisher=[[Internet Engineering Task Force|IETF]] }}</ref>

Modern clients may use the ESMTP extension keyword <code>SIZE</code> to query the server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute.<ref name=parms>{{cite web|title=MAIL Parameters|url=https://www.iana.org/assignments/mail-parameters/mail-parameters.txt|publisher=IANA|date=14 February 2020|access-date=May 28, 2019|archive-date=May 28, 2019|archive-url=https://web.archive.org/web/20190528161544/https://www.iana.org/assignments/mail-parameters/mail-parameters.txt|url-status=live}}</ref>

Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the <code>HELO</code> command with the <code>EHLO</code> command.
 <span style="color: gray">S:</span> <span style="color: blue">220 smtp2.example.com ESMTP Postfix</span>
 <span style="color: gray">C:</span> EHLO bob.example.org
 <span style="color: gray">S:</span> <span style="color: blue">250-smtp2.example.com Hello bob.example.org [192.0.2.201]</span>
 <span style="color: gray">S:</span> <span style="color: blue">250-SIZE 14680064</span>
 <span style="color: gray">S:</span> <span style="color: blue">250-PIPELINING</span>
 <span style="color: gray">S:</span> <span style="color: blue">250 HELP</span>
Thus ''smtp2.example.com'' declares that it can accept a fixed maximum message size no larger than 14,680,064 [[Octet (computing)|octets]] (8-bit bytes).

In the simplest case, an ESMTP server declares a maximum <code>SIZE</code> immediately after receiving an <code>EHLO</code>. According to {{IETF RFC|1870}}, however, the numeric parameter to the <code>SIZE</code> extension in the <code>EHLO</code> response is optional. Clients may instead, when issuing a <code>MAIL FROM</code> command, include a numeric estimate of the size of the message they are transferring, so that the server can refuse receipt of overly-large messages.