Pop3Vscan is a transparent POP3 proxy, which can be used to scan all pop3 communication going through the router. You can for example use it to protect a small network by scanning all outgoing pop3 connections. Use with pop3vscan is quite easy. Warning : Pop3Vscan is no longer maintained. You should prefer p3scan http://p3scan.sourceforge.net/ which is a fork.
I used Debian to do it, so I just installed pop3vscan from testing (apt-get -t testing install pop3vscan). You can try to fetch pop3vscan from your distribution or compile according to howto.