Currently, e-mail clients is almost completely IMPA4 + extensions. There are proprietary protocols used by Microsoft and GMail. JMAP is a modern non-proprietary alternative.
JMAP is evolved from FastMail’s own proprietary protocol, which they opened. But unless it’s an IETF RFC, nobody is interested.
Standars are constraints. They are solid, a great basis for a test suite. But becuase they are fixed, they take a lot longer to write than the software itself. IETF is a great breeding ground: that exposure and discussion has made the JMAP standard both simpler and more powerful than the FastMail protocol was.
In addition to JMAP, there is also a standardized extension of IMAP in the work: EXTRA. It’s a collection of unpublished extensions and idea.
See https://jmap.io who is using it.
Cyrus IMAP server already has JMAP support. fastmail.com offers a free e-mail service.