Mail Filtering With Mail::Audit
   An article describing the usage of the Mail::Audit Perl Module.
   http://simon-cozens.org/programmer/mail-audit.html

Sendmail::Milter
   A Perl module that allows programmers write mail filters for sendmail.
   http://sourceforge.net/projects/sendmail-milter/

The Evolution of Perl Email Handling
   Article by Simon Cozens on perl.com, with an overview of the main modules in the Mail:: and Email:: namespaces.
   http://www.perl.com/pub/a/2004/06/10/email.html