Policyd is an anti-spam plugin for Postfix (written in C) that does :

 o Greylisting,
 o Sender-(envelope, SASL or host / ip)-based throttling
   (on messages and/or volume per defined time unit),
 o Recipient rate limiting,
 o Spamtrap monitoring / blacklisting,
 o HELO auto blacklisting
 o HELO randomization preventation,
 o Sender mail addresses or domain Sender blacklisting

WWW: https://wiki.policyd.org/