This is a non-starter because it doesn't, and can't, work the way you expect it to. Email can be thought of as an encrypted communications protocol where the p

This is a non-starter because it doesn't, and can't, work the way you expect it to. Email can be thought of as an encrypted communications protocol where the parties can negotiate down to plain text. In other words, it isn't encrypted communication, it's bare communication over which the user might choose to send encrypted information. Something with the ergonomics of email, but built over a platform with enforceable E2E such as Matrix, would be quite welcome. But it can't be an extension of mail protocols, or reuse the email address space, so it won't be email.