This specification defines a very small mechanism to expose JMAP via HTTP (although JMAP uses HTTP already, it does so in a highly specialised way that is not accessible to most HTTP clients).

As a general comment, I wonder whether it's helpful to have "REST" in the title, since this is clearly a minimal API that happens to be exposed over HTTP; it has more to do with RPC than REST. Perhaps "JMAP HTTP Resource", "JMAP HTTP Interface" or similar?

Two specific issues to consider:

* Section 1.3 seems to implicitly reinvent RFC 6570. Have you considered using that syntax instead?

* Section 2 always uses POST. Is it possible to map some calls to GET to obtain benefits such as caching, idempotence, retry ability, etc.?