As a Proton mail user,
I want all incoming mail sent to my primary Proton username (email address) to automatically bounce back to the sender as "undeliverable" (as if the account does not exists), unless the email originates from an official Proton mail domain,
So that I an keep my primary account username entirely hidden and free of clutter, forcing all external correspondence (i.e. everything except for official Proton correspondence) to go through my managed aliases or Proton Pass aliases

*I understand I can have something similar by creating a custom Sieve with reject or discard, but it is not the same as having Proton at the mail gateway level automatically reject all non-official-proton mail (i.e. a hard bounce; which in effect unlike the Sieve implementation would lead a sender to believe the email address does not exists)

**This problem in part exists because proton usernames by default have an email addresses with the same username linked to them (if this were not the case then this feature would not be needed because there wouldn't necessarily be an email address that matches the given username)

***There must be strict whitelisting if this feature is implemented: Emails from Proton administrative, billing, an system alerts) should bypass this block and be delivered.

****The feature could for example be modifiable to allow emails from other proton users (i.e. proton.me, protonmail.com, proton.me, etc.) if the user chooses, which would bypass the block for them but reject any mail from any non-proton domain