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A common question regarding SpamAssassin's rules is, why aren't they kept secret? Doesn't publishing the rules alert the 'bad guys', causing them to change their spam patterns to evade the rule?
This is true, but only to a degree. In a way, this is an example of the \[http://en.wikipedia.org/wiki/Security_through_obscurity 'Security through obscurity'\] fallacy. Wiki Markup
Spammers aren't all that smart
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In other words, even though they had no access to the source code or published details of AOL's system, they could reverse-engineer enough details to get around it.
A Test Case
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Here's an analysis of \[http://taint.org/2005/08/06/024026a.html the change in effectiveness of the MIME_BOUND_DD_DIGITS rule\] over time, during 2004 and 2005.CategoryFaq