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The Internet Mail Service supports SMTP Service Extensions (ESMTP). ESMTP is an extension to SMTP that defines a means whereby an SMTP server can inform an SMTP client of the extensions it supports. ESMTP does not require modification of existing SMTP client or server configuration.

For more information on ESMTP, see RFC 1869, "SMTP Service Extensions."

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 that the server supports. If the server does not support ESMTP, an error response is generated.

The following is an example of a successful response to the EHLO command from the server as requested by the client The server supports the SMTP Delivery Status Notification (DSN) and Size of the Message (SIZE) commands.

Server: 250 XVR
Server: <wait for connection on TCP port 25>
Client: <open connection to server>
Server: 220 SMTP service ready
Client: EHLO
Server: 250- says hello
Server: 250-DSN
Server: 250-SIZE
Server: 250-Xauth
Server: 250-Xexch50

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