Work on JMAP likely started before GraphQL was publicly released.
Re:JMAP: Like IMAP but Not Really
by ForHackernews on Friday January 25, 2019 @04:11AM
(#18996623)
I think Fastmail are the primary ones behind JMAP: https://fastmail.blog/2014/12/23/jmap-a-better-way-to-email/
Re:JMAP: Like IMAP but Not Really
by jasonmunro on Friday January 25, 2019 @04:50AM
(#18996785)
Yep. Also I think they are using it for a subset of customers. While JMAP support in Cypht is still new, so far it's working just like IMAP, but my testing is limited since I just wrote the integration :)
[0] https://jmap.io/index.html