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Practical Common Lisp by Peter Seibel

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CHAPTER 23Practical: A Spam Filter

In 2002 Paul Graham, having some time on his hands after selling Viaweb to Yahoo, wrote the essay "A Plan for Spam"1 that launched a minor revolution in spam-filtering technology. Prior to Graham's article, most spam filters were written in terms of handcrafted rules: if a message has XXX in the subject, it's probably a spam; if a message has a more than three or more words in a row in ALL CAPITAL LETTERS, it's probably a spam. Graham spent several months trying to write such a rule-based filter before realizing it was fundamentally a soul-sucking task.

To recognize individual spam features you have to try to get into the mind of the spammer, and frankly I want to spend as little time inside the minds of spammers ...

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