This software block SMTP sessions used by e-mail worms and viruses on the NA(P)T
router. It depends on netfilter framework of Linux, or ipfw on FreeBSD.

It acts like proxy, intercepting outgoing SMTP connections and scanning session
data on-the-fly. When messages is infected, the SMTP session is terminated. It's
to be used (mostly) by ISPs, so they can eliminate infected hosts from their net
work, and (preferably) educate their users.

WWW: http://software.klolik.org/smtp-gated/