A few issues. PoW doesn’t prevent targeted attacks. As an attacker, I could target all big accounts, or just new users. Or just their DMs. 

Also hard to link PoW to visibility. Same hash cost whether a spam message is seen by one or 10,000. 
 
I’m also worried about PoW and phone batteries. 🪫

I’ve been thinking whether we could create a sponsor model. Would there be a way to onboard new users into the WoT through an existing user (their sponsor). A one-time onboarding step may be better than ongoing PoW for every message.