Protonmail could provide OPENPGPKEY DNS records as a means of distributing email encryption keys for their proton.me, protonmail.com, pm.me, and protonmail.ch addresses.

For external addresses, something akin to https://manpages.ubuntu.com/manpages/jammy/man1/openpgpkey.1.html could be provided to generate DNS records through the user interface, which the user can then register with their DNS providers.

https://www.rfc-editor.org/rfc/rfc7929.html