Thanks for the reference. But this misses out on another important tool already in use against spamming: White lists.

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.