I was interested in hosting my own mail server that provides a similar level of privacy for users as Protonmail, ie the server admin cannot read any emails, even those which are not E2EE with PGP. Is there a self-hostable solution to this?
I’m aware the server admin can’t read emails that were sent encrypted using the user’s PGP key, but most emails I get are automated emails from companies/services/etc without the option to upload a public key to send the user encrypted email. If you’re with a service like Protonmail, the server admin still cannot read even these emails.
well for e2ee you obviously have to let one e encrypt the data for the other e. (good luck with newsletters then) for usual services kindly asking them to support either s/mime or gpg for outgoing emails, that would at least make them know the wish, but good luck there too.
i think the already mentioned solution with encrypting incoming messages on your side just before mda to your inbox should be the closest possible to what op wants. one would need to check if the message is already encrypted and skip encryption for those.
if you only want the admin of that email (imap) server to not be able to read all emails, maybe placing a separate encrypting server (smtp+encrypt+forward) inbetween outside world and your email imap server could be a solution.
one should have a look into the logfiles too as some mailers might log message subjects and of course sender/recipients along with ip adresses of incoming/outgoing servers which the op might not want to be readable as well (i dont know protonmail that much)
also gpg IMHO allows for sign-then-encrypt hiding the signature within the encrypted data which could be wanted. also one might want to look exactly what parts of the messages contents and its headers are encrypted or plaintext on the server before feeling safe from the threat one wants to be protected from.