Extended SMTP (ESMTP) is an extended version of the Simple Mail Transfer Protocol (SMTP), with all extensions
being backward compatible with the SMTP protocol. The extended SMTP functions are identified by Extended. For example, with ESMTP, the client
isidentified by the Extended Helo (EHLO) command
instead of HELO, as with classic SMTP protocol.Important extensions include the use of 8-bit characters in the ASCII character set, instead of the 7-bit characters in SMTP, and the size limit of an email that the server will accept. With ESMTP, the same e-mail can be sent to several recipients
atthe same time
. The extensions are described in RFCs 1426 and 1427. However, since the current SMTP versions contain all the features of ESMTP, ESMTP no longer has any significance.