It's worth mentioning that hashcash has never been widely deployed for email spam prevention, and never will be. (As far as I'm aware its widest success was getting a modest reduction in scores in the default SpamAssassin ruleset at one point, which -- AFAIK -- was eventually rolled back because, while people are attracted to the philosophy of hashcash, nobody actually uses it.)<p>Why?<p>1) The supermajority of spam is sent through systems which can be operated by an attacker but which are not actually owned by them. This includes, most prominently, botted-up residential PCs. Prior to wide availability of always-on high speed Internet it included email servers which were insufficiently locked-down. Hashcash does not meaningfully affected the economics of botnets.<p>2) The most successful anti-spam measure in history makes direct use of a consequence of #1: since mail servers do not move in nature that much, and legitimate mail systems do not frequently mix outgoing spam and outgoing ham, you can make IP-based reputational systems. If a residential IP starts sending email in large amounts, assume spam. If a novel IP starts sending email, treat as suspiciously smelling ham until they've demonstrated sufficient history, but flip the spam bit if/when they get aggressive. If a new MSA springs up, ensure their industry veterans at the helm understand the importance of their anti-abuse team, and tell them explicitly that their IPs are dead if they don't.<p>This worked fantastically well. It's the primary anti-spam measure which protects your inbox. (No. I know you think Bayesian filters are. They're more expensive to operate at scale, are virtually unusable by the common-denominator email user, and don't solve any problem better than IP reputation does.)<p>3) Hashcash never caught on in part because the people who care most about spam also care most about sending billions of emails. "We make it economically unattractive to send billions of emails" is a non-starter for them. You can guess who I'm talking about by taking a look at any user database and observing what percentage of email addresses in it terminate with the top, oh, five domain names. (Interestingly, email is a P2P protocol at the server level which is best described as "All peers are equal, in that they will be equally squashed beneath the boots of our governing oligopoly if they misbehave.")<p>Note that none of hashcash's problems are solved by "And now it can be stored for later."<p>Source: My first engineering job was as an anti-spam researcher.