We are using a free spam filter VMware appliance called ESVA.  This is a beautiful product - I've been running this appliance about a year, support about 50 accounts.  I'll show off a little of the features..

We get no reports of false-positives.

We get less than 5 spam messages a month to the larger accounts.

We can easily blacklist and whitelist.

The product is FREE! (http://global-domination.org/ESVA)

Check out some stats: 

You can download our whole report for rule hits - this shows what percentages of spam hit what rules to make them designated as spam, very cool | DOWNLOAD

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