> For email uses, a textual encoding of a hashcash stamp is added to the header of an email to prove the sender has expended a modest amount of CPU time calcula

> For email uses, a textual encoding of a hashcash stamp is added to the header of an email to prove the sender has expended a modest amount of CPU time calculating the stamp prior to sending the email. In other words, as the sender has taken a certain amount of time to generate the stamp and send the email, it is unlikely that they are a spammer. I can see exactly why this wouldn't work - nowadays wouldn't spammers have enough processing power to just do this for every spam email? And how does this not slow down email processing for the end-user, sending an email now requires me to brute-force a hash, right?