This milter is responsible for making selective greylisting to certain IPs (dynamic ranges, fundamentally) and the result has certainly been very effective.
In the following graph, we see how the number of messages that are processed by the anti-spam engine (at this point, all the RBLs and SMTP tests have already been passed, so they are messages that happen to be processed by MailScanner and SpamAssassin) gives a significant drop from the start of said milter.
All this without a single false positive :-)
It is clear, as time has shown, that the ways to fight spam must go through this type of solutions. In fact, we will have to start seriously testing PostScreen, a new functionality that is already present in Postfix (from its version 2.8) and that will allow you to have protection ahead of the smtpd daemon. I promise post about it :-)
Author: Alvaro Marín Illera
Source: www.alvaromarin.com
