This filter started life in 2003 as a free way to filter spam at home.  It then migrated to my new job site (lead tech guy for a Microsoft Gold Partner) - my first week at the new job brought in spam galore.  I created the filter for home use, and wanted to impress the boss, so I brought it in and set it up at work.

My first attempt was done using instructions from Flakshack on an old P-II 300mhz with 512mb RAM and an 80gb HD.

The problem with the above setup is OpenBSD - sure it is secure, but it is also a pain to maintain.

So the filter then went to FreeBSD.  After a bit, VMWare Server free edition was released, and I created a FreeBSD VM, which I installed at over 30 business sites ranging from 5-400 users.  After a 24-hr freakout with portupgrade, I abandoned FreeBSD and moved to Fedora Core 5.  That wasn't the most stable of setups, however.

I recently scratched the entire machine, and started over with CentOS Linux 5.3.  It has proved to be very stable under heavy loads, it hasn't had any upgrade problems, and it should have security patch support to 2014.  That makes it ideal for production use. 

Other Sites used to create this filter:

Debian + MAIA

Postfix Anti-UCE Cheatsheet

Anti-SPAM Gateway

The supporting .vbs scripts were my attempts at making life easier for myself.  They are cobbled together from numerous sources on the web, so many I can't remember them anymore.  The script that extracts email addresses from Active Directory & gets it to the filter - that one was HUGE at cutting down problems.  Basically, a spam filter kinda sucks if it doesn't have a list of valid email addresses in it.