alt Hacker NewsEmail isn't a protocol. SMTP, POP, IMAP, JMAP are protocols.
GMail requires you to enable IMAP/POP to use it and uses proprietary auth protocol or use unsafe application passwords.
GMail IMAP also has a bunch of IMAP extensions that are unique to GMail.
Point is - primary interface to GMail is REST API that is (again) unique to GMail.
Fastmail was developing JMAP for a while, it's not gotten a lot of uptake (mostly because Fastmails primary mail partners like Gmail, AOL, Yahoo and Hotmail are all 20 year old legacy dot com companies...did Microsoft spend all day trying to get Fairchild Semiconductor to play nice with them? No, they did a worse is better DOS and the rest is history).
But email is a least common denominator, and like how plan9 failed to take over from unix bc unix inertia, JMAP or deltachat IM over email won't take over bc of network effect inertia, I suspect.