Stands for SMTP Service Extensions, an extension to the Simple Mail Transfer Protocol (SMTP) protocol that lets receiving SMTP hosts inform sending hosts of the extensions that it supports. ESMTP requires no modification of the SMTP configuration of either the client or the mail server.
Instead of beginning the session with the helo command, the receiving host issues the ehlo command. If the sending host accepts this command, the receiving host then sends it a list of SMTP extensions it supports. The sending host then knows which SMTP extensions it can use to communicate with the receiving host.
The Internet Mail Service for Microsoft Exchange Server version 5.5 supports ESMTP. The ESMTP standard is defined in Request for Comments (RFC) number 1869.