I know this is now a ways out from when the original post went up, but I have to agree with John Gardeniers regarding Mailcleaner.
I've used Mailcleaner now for roughly 4 years. The initial edition wasn't as flexible to modification, but it was pretty solid anyway. Around a year ago, I got ahold of Mailcleaner 2010, which is a complete re-write of Mailcleaner from the ground up. While the 2006 edition of Mailcleaner was built on a Debian v4 based engine, the newer 2010 release is based on Ubuntu Server, if I'm not mistaken. The older build was prone to breaking, but the newer build is so solid and feature filled that I haven't felt the need to do any under-the-hood modifications.
As is, Mailcleaner 2010 is by far the most solid and clean anti-spam solution I've used this side of a Barracuda Anti-spam / anti-virus Firewall. At my work, we use a Barracuda M600 appliance, designed to handle roughly 30 million email per day. We receive around 300,000 email on an average day, with roughly 7% to 10% of that being actual legitimate email. On the Barracuda, we use quarantining (which I abhor, but our management insists). On my personal domain, where I use Mailcleaner 2010 configured as a virtual server hosted on VMWare ESXi, I have Mailcleaner configured for LDAP address verification combined with tagging on suspected spam. All tagged spam is automatically delivered to my users' 'Junk Email' folder on our Exchange Server (2003), which automatically expires out the tagged messages after 30 days. This makes for a very, very low footprint for my Mailcleaner Anti-spam gateway, and with the auto-expiry of tagged email, it keeps my Exchange Server from overflowing with spam. The false positive rate is very, very low, and since I use tagging, even if a message receives a false positive, we don't have to worry about loosing that mail so long as we check in on our email responsibly... which all of my regular users do.
Anyways, having used a Barracuda anti-spam system, I had very picky expectations regarding an alternative for my own domain, since we don't have a corporate/government level budget to fund the purchase of appliances. All things considered, it would have been difficult at best to find a better solution than Mailcleaner 2010, because it seems to take a lot of influence from Barracuda, but it's not a straight rip-off of Barracuda's firmware. At the same time, it's much easier to setup than the Barracuda.
When I first started working my current job, my employers were using GFI's anti-spam engine (v10?). We had severe problems out of GFI because of how it handled blacklisting. I did the research and scored us the Barracuda M600. The Barracuda has been a great solution, because it allows for blacklisting by CIDR addresses, as well as a ton of other scanning techniques that work very well. So far, the only problems I've run into with the Barracuda are regarding false positives due to bad rules entered by those less experienced in combatting spam. The Mailcleaner 2010 Virtual appliance that I run requires much, much less interaction to achieve a near perfect solution.
Check it out if you get a chance. I think you'll be glad you did. :)
Share & enjoy!