SMTP extension
An optional SMTP capability, registered with IANA, that a server announces in its EHLO reply and a client may then use, such as SIZE, PIPELINING or SMTPUTF8.
What it means
RFC 5321 lets SMTP grow without breaking older software. Each extension has a keyword; the server lists the keywords it supports in its reply to EHLO, and the client uses an extension only if it was listed. An extension can add a command (STARTTLS, AUTH, BDAT), add parameters to MAIL FROM or RCPT TO (SIZE=, BODY=, NOTIFY=, REQUIRETLS) or change how replies are written (ENHANCEDSTATUSCODES).
IANA keeps the registry of SMTP service extensions. The ones a sender meets most are SIZE, 8BITMIME, PIPELINING, CHUNKING, STARTTLS, AUTH, DSN, ENHANCEDSTATUSCODES and SMTPUTF8. Reading a server's EHLO reply shows at a glance its size limit and whether it offers TLS.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes