If so, they haven't Open Sourced the implementation they're using (they have open sourced a JMAP-IMAP proxy that is pretty nice, but I probably wouldn't want to

If so, they haven't Open Sourced the implementation they're using (they have open sourced a JMAP-IMAP proxy that is pretty nice, but I probably wouldn't want to use a proxy in production). I don't recall what they've said about how they're using it, and if they're using it or some precursor to JMAP on their production systems.