[Lyrex]

In my personal experience I find that zone files work quite well as universal format for that. To pick up your Fastmail example: Fastmail could generate a matching zone file for your domain and let you download it. You could then upload it to any domain service provider that supports importing zone files.

It's obviously not as hassle-free than something like your oauth example, but it's using the infrastructure that is already there.

[63]

Incidentally, just an hour ago I was setting up a mail server on a Digital Ocean droplet, and had to manually copy and paste 20+ DNS entries because Digital Ocean doesn't support zone file upload (only download). So, the zone file seems like a good enough solution if only everyone would use it.