JMAP is good, IMHO

JMAP is not just championed by FastMail: they created it. Their webmail service uses it. (They also use and contribute to Cyrus; in a closely-related development, Cyrus supports JMAP.)

I read the RFCs a few years back. It seems to me to be a clever way to synchronize lists of lists between client and server over HTTP using JSON ... but I'm not an expert on this topic.

(Disclosure: I am a very satisfied customer of FastMail, and heartily recommend them to anyone who, like me, wants to never ever have to set up spam filtering again.)