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.
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I am using ORF ( It is based on DNS blacklists and other methods:
It is cheap, effective and have reasonable quantity of false positives.
Understanding Anti-Spam and Antivirus Functionality

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