Fortunately there is work being done on standardising the encryption of message headers when using PGP: https://datatracker.ietf.org/doc/draft-autocrypt-lamps-

Fortunately there is work being done on standardising the encryption of message headers when using PGP: https://datatracker.ietf.org/doc/draft-autocrypt-lamps-protected-headers/ https://datatracker.ietf.org/doc/draft-autocrypt-lamps-prote... It should be possible to go a step further, though, if mail providers offered a well-known reserved address like switchboard@domain which Alice could send an encrypted email to, and it would decrypt it to determine that the intended recipient was bob@domain. Ideally, Alice's mail provider would rewrite the sender headers to something generic too, relying on Bob to extract those from the email he decrypts. That way the sending server would only know the recipient server (not the local-part of the recipient address) and the recipient server would only know the sending server (not the local-part of the sender). Of course, this would make server-side spam filtering much harder, so it would benefit from some sort of bearer token system for bypassing such filters. The Autocrypt standard already uses special headers for signalling state between sender and recipient, but this header would require the mail provider to detect the new header and change their filtering rules based on it.