This is probably the solution to getting something like this off the ground. If the minimum PoW is zero (so not participating) and a >0 PoW is seen as a positiv

This is probably the solution to getting something like this off the ground. If the minimum PoW is zero (so not participating) and a >0 PoW is seen as a positive flag in spam filters. However I guess that most Spammers don't even setup DKIM etc. or fail it. So whoever would implement the PoW to send legitimate emails is probably doing other stuff already that get their emails flagged as legitimate.