The web3 answer here would probably be “use blockchain for identity” which is a hard thing to get right (understatement) when you consider “right to deletion” and various privacy laws, but imagine you could tie an email sender to some pseudonymous trail of transactions that prove it is a real economically transacting agent and not a bot.

I don’t think anyone is proposing to use blockchain for email? (I’m prepared to be wrong here but it’s certainly not a common position to hold). They don’t eschew all the old protocols for every task.