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What about at an enterprise level? We can spend on 1500 on a 50 seat spam filter program...or spend 1000 on a linux server to act as a spam and AV filter...and blacklists? Is there any good documentation on this subject?
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maybe you should consider signing up for a "black-hole" list to update your spam filtering.

http://www.google.com/search?hl=en&q=black+hole+list