If you are running a webserver you might have faced the problem
already: somewhere on your server is a vulnerable contact form or CMS
system written in PHP that gets abused by spammers to send emails
trough your server. If you have more than a few websites, it is a pain
to detect which of the sites is vulnerable and sends the spam emails.
This tutorial explains the installation of a small wrapper script which
logs email messages sent trough the PHP mail() function.