Hey cool - glad to see James being used as well. Hopefully the JMAP support that Linagora have worked on will mean that you can bring JMAP eventually too.
I hope you don't find the pain of diverging from the mainline to be too great. We kind of cheated there with Fastmail and Cyrus IMAP by merging all our changes back to the mainline, since there wasn't much other development happening.
Awesome - do keep in touch while you're doing it. We have some documentation up at jmap.io but of course seeing the challenges that people face as they try to implement is always good for improving the documentation for the next round.
(We're also working on JMAP for calendars and for contacts over in the IETF working groups - hoping to publish Calendars by the end of this year)
I hope you don't find the pain of diverging from the mainline to be too great. We kind of cheated there with Fastmail and Cyrus IMAP by merging all our changes back to the mainline, since there wasn't much other development happening.