Curtis:
> smtp_discard_ehlo_keywords = pipelining,silent-discard
>
> (Again, this time without the d.) Aside from a little extra bandwidth,
> would this cause outbound deliveries to go horrendously slower on a busy
> mail server?
The issue is NOT bandwidth (i.e. the width of a pipe). It's latency
(i.e. the distance between pipe endpoints). You can increase the
former, but you can't break the speed-of-light limit.
Pipelining reduces the number of TCP round-trip times, At a minimum
there will be 5 round-trip times (SYN-SYNACK, ACK-220, EHLO-EHLOREPLY,
DATA-DATAREPLY, ENDOFDATA-ENDOFDATAREPLY; the SMTP client does not
wait for final handshake completion). Disabling pipelining adds one
round-trip time for each MAIL FROM and for each RCPT TO command,
from 5 to 7 or more. Your SMTP server may spend 40% more time
delivering mail, depending on the number of messages per destination
(which reduces impact of DNS lookups), and on the number of recipients
per message.
Postfix has bug workarounds for CISCO PIX that are enabled automatically
when mail has been queued for 500s ore more. Maybe some of the
optimizations such as command pipelining could also be made
time-dependent.
Wietse
