This blog suggests bouncing messages from gmail.com users with a DSN error "The person you are trying to contact hasn't accepted Gmail's privacy policy. Please try sending the email from a regular email provider."
Allowing Protonmail users to enable a bounce like that for incoming messages would be an interesting way to let them assert their right not to be under Google surveillance.
Anonymous
This blog suggests bouncing messages from gmail.com users with a DSN error "The person you are trying to contact hasn't accepted Gmail's privacy policy. Please try sending the email from a regular email provider."
Allowing Protonmail users to enable a bounce like that for incoming messages would be an interesting way to let them assert their right not to be under Google surveillance.