[necovek]

Hum, maybe finally a good use of a distributed, append-only database (like blockchain)?

Reputation ledger where reported spam takes a lot off (say 100 units), sending an email costs little (1 unit), and possibly a way to earn more reputation by not having particular email be reported as spam (1.1 units back after 2 weeks for every sent email).

To enter the system, you need to start with at least 1000 units.