> looks to me like a full implementation of the RFC in rust

Only the client parts. And only the client parts its actually using. JMAP clients can be much simpler than servers. A JMAP server needs the whole protocol. JMAP clients only need to implement the parts they use. Servers also need to parse email message envelopes - which is way more difficult to do correctly than people think. JMAP clients can just use pre-parsed messages from the server.

Anyway, the code is here if you wanna take a look:

https://github.com/josephg/claude-mail

Claude put its JMAP API wrapper code in a child crate (confusingly also called jmap-client).