[arielcostas]

You can fetch a user's PGP public key via their HKPS endpoint, for example https://mail-api.proton.me/pks/lookup?op=get&search=username.... The one who apparently doesn't support PGP at all is Tuta.

Ideally, you'd be able to provide the service your key directly (you can do it in Sourcehut for example, IIRC), and they use that key without relying on a third-party server. Maybe using something like WebFinger could be a solution too, for automatic key discovery from a "trusted" party (the recipient's email server).