If you attach a signed monetary transaction cost using a decentralized smart contract transaction system to sending email to a federated host I hypothesize that

If you attach a signed monetary transaction cost using a decentralized smart contract transaction system to sending email to a federated host I hypothesize that spam would start dwindling. Email, IMO, largely grew into a centralized system due to the difficulties in combating spam. From the receiving end where filtering and blacklisting is a major system administration task. Self hosting also requires high quality SRE skills to combat servers from being compromised and used to flood the federated network with spam. Make the act of sending a message cost the send or money and I think spam goes away and self hosting becomes much simpler.