I have been reading the Hashcash paper which is referenced in the Bitcoin whitepaper. I would like to know what is the current state of the Hashcash protocol and what are the services that are actively using it?

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There is no serious usage, and has never been, of Hashcash.

Most obviously, the disparity between the resources of a spammer (who can rent a botnet of stolen compute resources, or use high powered servers and GPUs) is no match for a user sending an email from their phone with limited power available . This simply means that the difficulty of generating a "stamp" has to be so high that only spammers are able to send messages, which defeats the purpose.

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