Code is here: https://github.com/josephg/claude-mail

The JMAP client itself is hosted here: https://seph.au/claude-webmail/

I can't prove this but its a purely static web app. You need a jmap server to use it. If you use stalwart, set:

    server.listener.http.permissive-cors = true

or

    server.listener.https.permissive-cors = true

Then you should be able to put https://localhost:8080/ into the URL box. It should also work with fastmail, but I haven't tested it.