Pretty cool protocol. I've been following this for a couple of years. I had already written a library for an earlier version of JMAP (back then references worked differently / didn’t exist); and it was interesting to see how that improved in the IETF process.
I wrote a library [1] and a POC email client for Android [2] earlier this year. It takes a moment to fully understand a now fairly complex protocol but when you get the hang of it it becomes very powerful.
Sadly the server support isn’t really there yet. The support for Cyrus hasn’t been released yet (you need git) and some vital functionality like push [3] is still missing. Also no word from Dovecot yet.
[1]: https://github.com/iNPUTmice/jmap