Policyd is an anti-spam plugin for Postfix (written in C) that does
greylisting, sender-(envelope, SASL or host/ip)-based throttling (on
messages and/or volume per defined time unit), recipient rate limiting,
spamtrap monitoring/blacklisting, HELO auto blacklisting and HELO
randomization preventation.

Homepage:
http://www.policyd.org/
