I think that there is way Proton could be the best encrypted and private platform (never mind that any email sent from a non non-Proton account will not be E2EE and can still be read by third parts because it is stored on non-Proton servers). When a Proton user receives a non-E2EE email, say from a Gmail user, Proton must decrypt that email and have access to it in primary memory in plaintext. If Proton's servers are compromised and suffer from zombie memory attacks (even a trusted execution environment (TEE) running cubes OS would not be safe from this kind of attack only turning off all CPU software optimizations on either AMD or Intel CPU will remove this possibility) then an attacker could gain access to user's emails. Another issue could be that Proton could be compelled to turn over any plaintext emails it can read in the future. To prevent this Proton should never decrypt the non-E2EE emails. Instead take the private key for decrypting the non-E2EE email and encrypt it alongside the non-E2EE email message with the Proton user's public key. Then when the user wants the read that email they decrypt them email with their private key (on device of course) then now the Proton user's device now has the private key to the message and the encrypted message itself. Then the user's device has to decrypt the message and now the user can read the message without Proton ever seeing a user's plaintext message. Also please give users the choice to encrypt things like subject lines and date and time for calenders. Then if they user chooses to have these options encrypted then the only possible way for users to search for their emails will be on device when the device has been decrypted. Or the user's calendar can remain decrypted on their device but always be E2EE on the server. This way users can still sync with the server and their devices can still see all the same calendar events and users can still get notifications bc it's on device. And users love having choices especially the ones who love privacy.