I have been using pgp since the late 90 s . The problem is that while quite a few add on clientsfront ends exist for windows machines there is nothing for iOS . Ideally there would be an iOS email client that would have built in pgp but would be customizable to select the email service . Anyone logginghacking into the email server would see just the pgp encrypted text . All encryption and decryption would take place on the device itself . While protonmail does apparently encrypt and decrypt locally , and allows password protected messages to be sent users who are not in protonmail , its biggest risk assuming the system itself is secure , is reliability of the service . Gmail will not likely fail in in the foreseeable further , protonmail could easily fail or be forced into shutting down .