This is a great idea [your edit]!
And its already being developed by some (not protonmail though). See https://github.com/ehloonion/onionmx
And its already being developed by some (not protonmail though). See https://github.com/ehloonion/onionmx
mike@snake:~$ dig +short srv _onion-mx._tcp.grepular.com
0 5 25 grepularmmmiatj7.onion.
mike@snake:~$