Extended SMTP (Also known as Enhanced SMTP)

Even though the SMTP protocol is well defined (by RFC 821), several problematic issues were discovered with time.
One problem relates to message length. Some older implementations cannot handle messages exceeding 64KB. Another problem relates to timeout - if the client and the server have different timeouts, one of them may give up while the other is still busy, unexpectedly terminating the connection. Finally, in rare situations, infinite mail-storms can be triggered. For example, if host 1 holds mailing list A and host 2 holds mailing list B and each list contains an entry for the other one, then any message sent to either list will generate a never-ending amount of E-Mail traffic.

ESMTP (defined in RFC 1896, in year 1995) defines an extension format that allows adding new extensions to the basic SMTP to confront various issues like the ones mentioned above.

When a client wishes to use an extension, the conversation should be started with an EHLO command instead of the default HELO command. If the server supports ESMTP, than together with the OK response, a list of supported extension will be given as well. Otherwise, the server won't accept the EHLO command an error message will be sent. The following commands are dependent on the specific extensions.
Today, the support for the EHLO command in servers was made a "MUST," superseding the original HELO.