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.
