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The scores are assigned using a neural network trained with error back propagation (Perceptron). Both systems attempt to optimize the efficiency of the rules that are run in terms of minimizing the number of false positives and false negatives.
A list You can find all of the currently active rules and their assigned scores is at tests. scores in the Subversion repository under /trunk/rules or by downloading the latest published set using the sa-update tool.
You can help this system by providing statistics on your mail spool via NightlyMassCheck and RescoreMassCheck.
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