Obviously a subscriber to a mailinglist need to set the cost to 0 for the list sender.

The primary reason why a proof of work implementation for something like bitcoin has a huge environmental impact is because it is a winner take all system. You have thousands of machines trying to do the same thing but only one will get there first and claim all the reward, making all that other work that was done a waste of resources.

At some point 1:N becomes prohibitively expensive for the sender, but that doesn't mean that there is wasted work.

edit: To clarify, not really promoting the idea per say, just commenting on the proof of work statement.

No no no, this one is actually genius. The cost to send an email some one who doesn't know you (and has a real, functioning email) becomes too high to do casually in terms of the base resource (CPU time), I literally could not spam even if I wanted to without throwing absurd resources at the problem, which presumably would also make detection and correction easier if some of these elder-abusing rackets are more profitable than that. But! I could still send you a real email.

It would come down to numbers of course on whether it was actually a good idea, and one of those numbers would have to be "how much work needs to be done before your average spam calling scammer is no longer profitable", but as it stands if there is a number sufficient for that that still allows for regular emails, then the incentive to burn machine time on sending out mass-emails is effectively removed.