That would assume that all spammers use botnets or can easily obtain specialized hardware (which are mutually exclusive). It's still a deterrent and makes it n

That would assume that all spammers use botnets or can easily obtain specialized hardware (which are mutually exclusive). It's still a deterrent and makes it non trivial to be a spammer. Also even on botnets each node is large numbers of emails, so it increases the odds that the owner of the infected computer would notice all the work being done. Makers of specialized 'spam' hardware could potentially be targeted by law enforcement easier then dispirit spammers (especially if the hashing algorithm could be designed to be specific to email systems). I can see some arguments as to why it's not 100% effective, but I don't think it needs to be 100% effective to add additional value. That's why I'm amazed it's still not being done. Just like I'm amazed that we still don't have easy to use email encryption, yet demand it for the web (email traffic is far easier to eavesdrop on, and often far more sensitive).