Selective bounce feature for privacy
To protect privacy, proton user can make it appear that his account does not exist to all but those he prefers to get email from. I know hackers have attempted to identify accounts on mail servers by spraying them with likely mailbox names. The proposed option would allow a user to bounce incoming messages (SMTP code 550) unless the sender is on the list of people to whom he has sent messages in the past. If the the PM user wishes to review this list and remove certain people later, he should be able to do so. Optionally, allow limits within 50 recipients (free service) and 300 (paid service).
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.
Need I electrons? commented
Dunbar number. look it up, I believe it's somewhere between 125 and 250. I believe this may also be a concept that Google started with way back in 2004 while in beta testing. But you look at most social networks and it seems the people who need their dopamine drip prefer to rack up as many "acquaintances' as possible. Makes sense to revisit contacts every now and then as changing circumstances may require different contacts.
I would like to be able to suppress or spoof your bounce message. It is a metadata leak.