The filter is a small VM designed for 5-50 Microsoft Exchange mailboxes processing maybe 1000-3000 emails per day.

If you want a more enterprise grade setup, it would be pretty easy to accomplish.

• install one filter VM on each computer, so you can setup multiple MX records in your company DNS zones - good for redundancy - typically 1 VM per 1000 users is sufficient - each VM should have 2 processors assigned and 2gb RAM assigned, and you will need to adjust Postfix & Amavisd to spawn a lot more child processes
• each filter can point to a single back-end MySQL database server - remember that MySQL runs just fine under Windows - figure a quad-core processor with 8gb RAM, 64-bit OS and the 64-bit version of MySql 5.1 - this will handle a LOT of transactions, like 4 or 5 Virtual Machines processing over 100,000 emails per day - the MySQL server should have gigabit connectivity to all the spamfilter VMs that you setup
• put the virtual machines in a DMZ, with access to only the Exchange SMTP port (to deliver email) and the mysql TCP port (to perform database transactions), plus allow HTTP access to the filter(s) from the inside of your network
• you can even setup a mysql cluster under windows - that would eliminate any single point of failure in the spam filtering subsystem of your enterprise
• setting up both DSL and cable modem internet connections (a Sonicwall Firewall can handle these) eliminates the internet connection as a single point-of-failure also

If you want more info, drop me a line.