> Mailing lists and other legitimate email uses would be affected
The receiving server could require proof-of-work for the first message from a sender to a given receiver, but not for subsequent messages. If spammers try to abuse this, it's easy for the user to click "block this sender"; if the spammer changes their sending address, they have to do a new proof-of-work.
> Requires immediate total cooperation from everybody at once
It could be introduced gradually: if a proof-of-work is provided, then the message is allowed to skip the spam filter. Small mailserver operators often have trouble with their messages being caught in spam filters by GMail and other large webmail providers; even if GMail alone started accepting proof-of-work, that would probably be enough to convince a lot of senders to start generating proofs-of-work, which would drive adoption.
> Lack of centrally controlling authority for email
This is no more difficult than any other backwards-compatible extension to an existing Internet standard.
> Public reluctance to accept weird new forms of money
OP's post didn't mention a weird new form of money. The original HashCash proof-of-work proposal included a weird new form of money, but it's easy to imagine a proof-of-work system that doesn't.
> Sending email should be free
Suppose the proof-of-work cost is calibrated such that it costs about $0.001 per email. That would be enough to destroy spammers' margins, but the vast majority of legitimate users wouldn't mind paying that.
> Ideas similar to yours are easy to come up with, yet none have ever been shown practical
I kinda see the point here... But I'd like to understand _why_ it isn't practical, because none of the arguments make sense to me.