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hits since nov 03

SpamAssassin hashcash support

Hashcash support is in SpamAssassin (3.0). Details of how to configure it are given here in the spamassassin 3 docs.

Hashcash support was is already in the developer release 2.7, see usage notes (under "Other Installation Notes" which describe how to use hashcash as a user to add additional recipient addresses. (These would be for addresses that you have forwarded to the pop box that is being processed by SpamAssassin, and addresses of mailing lists you are subscribed to.)

The SpamAssassin approach to integrating hashcash is based on the SpamAssassin scoring model. For each spam-like trait of an email some scoring is added, if some threshold is reached the mail would be considered spam. If hashcash is present some bonus (negative) score is added making the mail less likely to be falsely identified as spam, making SpamAssassin much more forgiving of spam trait keywords in mail sent with hashcash stamps.

The SpamAssassin wiki refers to the bonus points (bottom of page):

To track the hashcash integration, and see more details of the scoring system see the bugzilla bug tracking the feature.