user90855: 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?

user24925: 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. 

90855: <p>I have been reading the <a href="http://www.hashcash.org/papers/hashcash.pdf" rel="nofollow noreferrer">Hashcash</a> 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?</p>
 24925: <p>There is no serious usage, and has never been, of Hashcash. </p>

<p>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. </p>
