Just came across this one, might be interesting. It sounds similar to postscreen's functionality:
EuroBSDCon 2009: FreeBSD kernel protection measures against SMTP DDoS attacks, by Martin Blapp http://people.freebsd.org/~mbr/ http://www.ukuug.org/events/eurobsdcon2009/papers/BSDCON09-SMTP-DDoS-Final.pdf http://www.disruptiveproactivity.com/other/eurobsdcon2009/smtp-ddos-protection.mp3 Mark
