In a nutshell, the idea is to assign an additional secret key (salt) to each email user account. The email sender uses the salt and the message contents to calculate a hash value and adds that hash value as a new email header. For each email domain a verification server is registered in the DNS that can be contacted to verify the authenticity of messages that contain a hash value in the email headers. An email recipient can contact a verification server and filter incoming messages based on the verification response. As soon as multiple email recipients report that a sender is sending spam, the sender's salt gets changed, and future verification requests for messages that used the older salt will fail and such messages can be rated as Spam.
For a full description of how this could work please see http://kuix.de/spamsalt/
I'm looking forward to your feedback.
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