ESMTP
Extended SMTP: SMTP with service extensions, where a server answers EHLO with a list of extra capabilities, such as SIZE, PIPELINING or STARTTLS, that the client may then use.
What it means
The extension framework was first defined in 1993 (RFC 1425, last revised on its own as RFC 1869) and is now part of RFC 5321. A client that opens with EHLO instead of HELO is asking which extensions the server supports; the server replies with a multi-line 250 response, one keyword per line, and the client uses only what was announced. Old and new software can therefore keep talking to each other.
You meet the word in Received headers. A hop written with ESMTP used EHLO, and RFC 3848 adds ESMTPS for a hop that used STARTTLS, ESMTPA for one where the client authenticated, and ESMTPSA for both. Reading those keywords down the Received headers is a quick way to see which hops of a message were encrypted.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 2.2: The Extension Modelrfc-editor.org
- RFC 1869: SMTP Service Extensionsrfc-editor.org
- RFC 3848: ESMTP and LMTP Transmission Types Registrationrfc-editor.org
- IANA, Mail Parameters registry (SMTP Service Extensions)iana.org
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