How ESMTP Works

An SMTP client supporting ESMTP starts an SMTP session by issuing the EHLO command instead of the HELO command. A successful response issues a list of SMTP extensions the server supports. If the server does not support ESMTP, an error is generated.

The following is an example of a successful response to the EHLO command from the server abc.def.ghi.com as requested by the client rst.uvw.xyz.com.

Server: 220 abc.def.ghi.com ESMTP Server (Microsoft Exchange Internet Mail Service) ready
Client: EHLO rst.uvw.xyz.com
Server: 250-abc.def.ghi.com supports the following extensions:
Server: 250-XEXCH50 [Exchange to Exchange data transfer]
Server: 250-HELP
Server: 250-VRFY
Server: 250-SIZE 0
Server: 250-ETRN
Server: 250 DSN
Server: 250 AUTH=LOGIN [SASL clear text password authentication]
Server: 250 TLS [transport level security]

For more information on ESMTP, see RFC 1869, "SMTP Service Extensions" and RFC 1870, "SMTP Service Extension for Message Size Declaration."