Support for JMAP format

[mhalano]

JMAP is a new format made to replace IMAP. It's more modern and with more features, like optimized attachment management. Here is the specification: https://jmap.io/. I think it's in the beginning yet, but we could consider thinking about support.

[keithbowes]

Well, offlineimap doesn't actually support IMAP at all. That's handled by imaplib2. I'd assume for JMAP support, someone would need to fork imaplib2 and make any changes necessary to make it work with JMAP (or create a new module that's API-compatible with imaplib2, with only class names that are different). Probably the biggest thing to get anyone to spend the time to implement it would be if there's actually any email provider that uses JMAP. It seems to be like POP4: a "standard" that's been around for years but hasn't gone anywhere and isn't on any track to become sanctioned by any recognized standards body. Plus, from the best I can tell, JMAP is only used internally by fastmail (it's a representation of how mail is stored on the server) but access by email clients is done through IMAP or POP.