While we at Nylas did build our APIs to help with IMAP troubles, we also handle Gmail (via IMAP) and Exchange as well. We'd be happy to add JMAP support to our APIs if it takes off since we're committed to 100% coverage of email mailbox accounts (including calendar via CalDav parsing).
(If the Fastmail people are reading this, feel free to contact me.)
I used Nylas a little while ago, loved the N1 client. But had to stop due to restrictions on emails being stored off site. Is there an option for people wanting to use Nylas where they have restrictive security policies on where email can be stored?
(If the Fastmail people are reading this, feel free to contact me.)