Yes, in theory PoW makes mass mailing very expensive and thus directly hurts spammers that are not on the common anti-spam white lists. Organizations that need to mass mail users for legitimate reasons are however most likely already on anti-spam white lists and thus need not to do a very hard PoW hash or none at all as they are already proven to not be spamming.
So this paper's conclusion is not the real deal.
But having such list will make email effectively centrally controlled. I would hope we never end up in this situation. The current system is bad, but making it centralized is even worse - just read what happens to businesses when Facebook/google bans them.
Of course some of the larger services may offer a default white list for common domains.
I suppose you can have a special system where you have a browser plugin which automatically generates some sort of subscription token and pastes it alongside email into subscription form.. but this will need cooperation from every mail-sending app, and will not handle non-web uses cases at all...
Abstract: ”A frequently proposed method of reducing unsolicited bulk email (“spam”) is for senders to pay for each email they send. Proof-of-work schemes avoid charging real money by requiring senders to demonstrate that they have expended processing time in solving a cryptographic puzzle. We attempt to determine how difficult that puzzle should be so as to be effective in preventing spam. We analyse this both from an economic perspective, “how can we stop it being cost-effective to send spam”, and from a security perspective, “spammers can access insecure end-user machines and will steal processing cycles to solve puzzles”. Both analyses lead to similar values of puzzle difficulty. Unfortunately, real-world data from a large ISP shows that these difficulty levels would mean that significant numbers of senders of legitimate email would be unable to continue their current levels of activity. We conclude that proof-of-work will not be a solution to the problem of spam.”