If your machine receives backscatter mail to random addresses,
configure Postfix to reject all mail for non-existent recipients
as described in the
If your machine runs Postfix 2.0 and earlier, disable the "pause
before reject" feature in the SMTP server. If your system is under
stress then it should not waste time.
# Not needed with Postfix 2.1 and later.
smtpd_error_sleep_time = 0