If you built a client on top of email, you could add PGP encryption as part of the service. Getting and accepting friend requests performs the key exchange handshake needed and voila! encryption. There's some nuances here that would need to be addressed, but nothing particularly showstopping.

Governments/ISP/School/Company could ban obvious encrypted SMTP message.

Encryption should be done in a obfuscated handshake way. That's what this paper lacks (and this really matters.)

Also if you have your own IMAP/SMTP server running, you should really think about some kind of TOR-like relay network to hide your server IP.