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: