With Postfix versions 2.0 and earlier, the
pauses before reporting an error to an SMTP client. The idea is
called tar pitting. However, these delays also slow down Postfix.
smtpd(8) server replies slowly, sessions take more time,
so that more
smtpd(8) server processes are needed to handle the
load. When your Postfix
smtpd(8) server process limit is reached,
new clients must wait until a server process becomes available.
This means that all clients experience poor performance.
You can speed up the handling of
smtpd(8) server error replies
by turning off the delay:
# Not needed with Postfix 2.1
smtpd_error_sleep_time = 0
With the above setting, Postfix 2.0 and earlier can serve more
SMTP clients with the same number SMTP server processes. The next
section describes how Postfix deals with clients that make a large
number of errors.