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 Warning : Pop3Vscan is no longer maintained.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.

The important part of configuration is this

virusregexp = .*: (.*) FOUND scanner = /usr/bin/clamdscan --disable-summary -i scannertype = basic demine

Don't forget to add iptables(8) rule, which will direct all pop3 traffic to pop3vscan first

/sbin/iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 110 -j REDIRECT --to-port 8110

Change eth1 to your local network's interface name (all request coming from this network interface will be directed to pop3vscan).