There already is one, written by the people behind JMAP even, with a public demo instance at https://proxy.jmap.io/ https://proxy.jmap.io/ (don't use the demo w

There already is one, written by the people behind JMAP even, with a public demo instance at https://proxy.jmap.io/ https://proxy.jmap.io/ (don't use the demo with your own credentials of course, host it yourself)