Put this in problems because it applies to Linux
and BSD. At least I think it applies to Linux too.
Someone correct me if I'm wrong.

You can have localhost and all your buddies and
sendmail too in /etc/hosts.allow but if you have
an /etc/hosts.deny file that reads

ALL: ALL

tcpwrappers and denials are going to be scattered all
through your /var/log/maillog, & you're not going to
get any mail except locally, like if root has a .forward
file for you to get system messages if you're an asst
admin.