gnus: Anti-spam Hashcash Payments

 
 9.17.6.4 Anti-spam Hashcash Payments
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  -- Variable: spam-use-hashcash
 
      Similar to ‘spam-use-whitelist’ (SeeBlacklists and
      Whitelists), but uses hashcash tokens for whitelisting messages
      instead of the sender address.  Messages without a hashcash payment
      token will be sent to the next spam-split rule.  This is an
      explicit filter, meaning that unless a hashcash token is found, the
      messages are not assumed to be spam or ham.