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In addition, we expect that spammers are adapting, as they always have to each new form of filtering. Several spammer countermeasures have already been seen in use, aimed at Bayesian classifiers. SpamAssassin's 'wide-spectrum' approach is designed to catch these evasion techniques.
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Swapping one form of load for another
A related criticism is that SpamAssassin is heavyweight, requiring more system load to classify mail than statistical filters do.
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