"email" isn't even a single protocol. Even just talking about SMTP you'd run into lots of "common edge cases" where different servers respond to errors differently, a lack of TLS uniformity, variations in auth, variations in host detection, etc. And that's before you take collection into account (POP3, IMAP, IMAP+, ActiveSync, etc).

Email is a mess of kludges and frankly could benefit from a ground up re-implementation. In fact many have tried. But like with creating a decentralised social network, it doesn't matter if you have an arguably better implementation if you don't generate the snowball effect attracting users to your "better" platform.