[deleted] It is because Proton's encryption works like Posteo's encryption on entry with PGP ? Activate it for Posteo provides the same level of privacy/security as Proton right ?
ProtonMail stores the mailbox with standard encryption (AES??) set-up so that Proton cannot let the contents be decrypted on their systems, nor would they ever know the keys to decrypt said mailbox. Posteo also store the mailbox this way, but decrypt the contents on their systems when accessed by the user (this is the flaw).
The PGP in Posteo and ProtonMail is used for end-to-end encryption between emailing one user to another. Using PGP or another end to end encryption method for emails will make Posteo unable to read the contents during an interception, and would make Proton unable to see incoming emails from services that aren't Proton's. Posteo makes you do it manually in all cases, while ProtonMail automates the PGP encryption for you on their service, this is why emailing another ProtonMail user is automatically end-to-end encrypted.
Big issue with PGP is it isn't well designed, you have to manually do the key transfers between others before communicating on the Email platform. PGP encryption has to be done with consent of both parties, which is practically impossible for emails from a company, and is less possible since most people just won't spend their time learning how to make a key and do it themselves. This is again going back to where there is a big flaw with email not being end to end encrypted.
While ProtonMail mitigates this by automating the PGP encryption/decryption process, it is only just a small patch fix for a variety of issues email has entirely, since not everybody uses ProtonMail or PGP