Standalone Bayesian spam filter proxy
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SpamBayes is not a single application. The core code is a message classifier, however there are several applications available as part of the SpamBayes project which use the classifier in specific contexts.
For the most part, the current crop of applications all operate on the client side of things, however, a number of people have experimented with using SpamBayes on mail servers to classify incoming mail for multiple users.