Extensions of the SMTP protocol led to the development of Extended SMTP (ESMTP) . The extension principle is specified in the RFC 1869 standard. The main problem concerning any extension is in its backward compatibility. In this case, the creators of ESMTP came up with a very smart solution.
While the SMTP protocol usually begins its dialogue with the HELO command, ESMTP uses the EHLO command. The server’s reply will be either of the following:
- The client must have made a mistake in the command name. The client will immediately realize that the server is SMTP-only and carry on with the
HELOcommand. - Status code 250 (shows that everything is OK). The client will immediately notice that the server is of ESMTP type. Furthermore, the server’s reply will contain the list of extension commands it supports:
EHLO libor.computer.org
250-dns.terminal.cz Hello libor.computer.org, pleased to meet you
250-EXPN
250-VERB
250-8BITMIME
250-SIZE 8388608
250-DSN
250-ONEX
250-ETRN
250...