A lot of “degoogling” talk has discussed how to degoogled email in a graceful way (Gmail has huge market share). Let me find that community.

Edit: https://sh.itjust.works/post/3044652

Edit: based on that post, you probably want fastmail to selfhost. Now the official API of mail clients is jmap, so it may not be “rest” and you may not be able to find a “reliable” rest api

Edit final: check this out https://en.m.wikipedia.org/wiki/Comparison_of_mail_servers And I still don’t know what ones have a rest api.