Freeware: SpamPal
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SpamPal sits between your email program and your mailbox, checking your email as you retrieve it. Any email messages that SpamPal considers to be spam will be "tagged" with a special header; you simply configure your email client to filter anything with this header into a separate folder and your spam won't be mixed up with the rest of your email anymore!" I have not personally used this product. I would like to test it in the near future.