I reached-out to the Postfix mailing list and one of the list's more accomplished contributors recommended strongly that I swap the order of two configuration directives, which are set "incorrectly" by default (I don't know whether by Ubuntu or by ISPConfig):
I strongly suggest that you swap the order of the following
two rules in main.cf:
This should be:
Many open relay problems are caused by having an access table
If this Postfix misconfiguration issue is, in fact, at the root of this problem, I'm surprised that others have not had similar experiences with flooded Postfix queues. I'm still not convinced that these two configuration values are the problem. Only time will tell.
I also modified my fail2ban configuration so that "relay access denied" offenders are banned (the appropriate regular expression already exists in filter.d/postfix.conf
, the Postfix section now reads:
enabled = true
ignoreip = 127.0.0.1/8
port = smtp,ssmtp
filter = postfix
logpath = /var/log/mail.log
bantime = 3600
# service fail2ban reload
I'll see how these changes affect the observed behavior and update this thread when I know more.