Resource for Anti-spam on exchange server

There are many anti-spam appliances, add-ons for exchange server 2007/2010. I am relatively new to this and trying to get a foundational understanding of how Anti-spam works on exchange server. Does anyone have any material that will give me a generic overview. We are using Brightmail and Pure message for our environments and trying to get a basic understanding more so than a vendors overview of the features they offer.
techdriveAsked:
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als315Commented:
I am using ORF (vamsoft.com). It is based on DNS blacklists and other methods:
http://www.vamsoft.com/lp.asp#efficient
It is cheap, effective and have reasonable quantity of false positives.
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E_T_BCommented:
Understanding Anti-Spam and Antivirus Functionality

http://technet.microsoft.com/en-us/library/aa997658.aspx
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