Make a new protocol for this decade, that isn’t email.
HTTP is supported nearly everywhere SMTP is. Just build something over that, and this time around make sure to avoid SPAM bullshit.
People shouldn’t be able to just message you based on your address. They receive a capability to email you. People can be empowered to give out your capabilities. If a particular such branch leads to spam, you simply cut off that branch and boom, no new user can reach you with that capability anymore. Hashes of Public keys can identify users.