JMAP is a radically different beast. The similarities between this thing’s JSON format for sending and JMAP’s Email data type are superficial only: they’re both JSON and are representing the same thing, so it should be no surprise that they look similar. But that’s a quite tiny part of what JMAP is: JMAP is an object synchronisation protocol. (And this is why JMAP so much more complex than the typical REST API. And why I prefer it so much.)
I also think the JSON here is only for sending, not for receiving—I presume that you’ll receive the MIME message, because otherwise you’d be throwing away all kinds of essential information.
All of this gets you basically nowhere along the path to JMAP, and achieving a JMAP endpoint would be a lot of effort. This project doesn’t look to be at all suitable as a base for such an endeavour. Things like sorting (e.g. newest first), querying (e.g. emails from so-and-so) and JMAP’s state management (so the server can tell you “something changed” and you can ask the server to tell you what changed since x, rather than needing to throw everything away and start again) don’t work well within the design of this system—you need to store lots of extra details along the way, maintaining indexes and other such things.
For such an endeavour, I would instead recommend either wrapping an existing mail server in serverless voodoo (much of which I expect to be not too hard: you’re essentially just replacing ingress and egress and not running it as a daemon; but there will be architecturally difficult parts like getting push channels working probably), or starting new mail server software from scratch designed to be able to work serverless.
(I work for Fastmail on our webmail. I have general knowledge of how mail servers work internally, but little specific knowledge; for example, I have no idea how amenable Cyrus, which we use and develop, would be to serverless packaging.)
