PIPELINING
An SMTP extension (RFC 2920) that lets a client send several commands in one batch without waiting for each reply, saving round trips.
What it means
Without pipelining every command waits for its reply, which adds a full network round trip per recipient. With it, a client can send MAIL FROM and all its RCPT TO commands together and read the replies afterwards, in order. RFC 2920 allows RSET, MAIL FROM and RCPT TO anywhere in a batch, but EHLO, DATA, VRFY, EXPN, TURN, QUIT and NOOP only as the last command, since their outcome changes what the client must do next.
A client may pipeline only after the server has announced the extension. Postfix can reject clients that send commands ahead of time without that announcement, a setting it describes as stopping bulk mail software that misuses pipelining. Pipelining must also pause at STARTTLS until the new EHLO: Gmail rejects commands pipelined after STARTTLS.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2920: SMTP Service Extension for Command Pipeliningrfc-editor.org
- Postfix configuration parameters, postconf(5)postfix.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
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