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Postfix Documentation
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What SMTP connection caching can do for you

With SMTP connection caching, Postfix can deliver multiple messages over the same SMTP connection. By default, Postfix 2.2 reuses an SMTP connection automatically when a destination has high volume of mail in the active queue.

SMTP Connection caching is a performance feature. Whether or not it actually improves performance depends on the conditions:

  • SMTP Connection caching can greatly improve performance when delivering mail to a destination with multiple mail servers, because it can help Postfix to skip over a non-responding server.

  • Otherwise, the benefits of SMTP connection caching are minor: it eliminates the latency of the TCP handshake (SYN, SYN+ACK, ACK), plus the latency of the SMTP initial handshake (220 greeting, EHLO command, EHLO response).

  • SMTP Connection caching gives no gains with respect to SMTP session tear-down. The Postfix smtp(8) client normally does not wait for the server's reply to the QUIT command, and it never waits for the TCP final handshake to complete.

  • SMTP Connection caching introduces some overhead: the client needs to send an RSET command to find out if a connection is still usable, before it can send the next MAIL FROM command.

For other potential issues with SMTP connection caching, see the discussion of limitations at the end of this document.

Postfix Documentation
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