Eigenpoll : Spam fighting software : MIMEDefang 

MIMEDefang is a framework for filtering e-mail. It uses Sendmail's "Milter" API, some C glue code, and some Perl code to let you write high-performance mail filters in Perl.

People use MIMEDefang to:

* Block viruses
* Block or tag spam
* Remove HTML mail parts
* Add boilerplate disclaimers to outgoing mail
* Remove or alter attachments
* Replace attachments with URL's
* Implement sophisticated access controls.

You're limited only by your imagination. If you can think of it and code it in Perl, you can do it with MIMEDefang.