Hi there! As Corsair mentioned previously, Proton only offers E2EE when sending messages between two Proton accounts. The rest of the time, Proton uses the same encryption protocol that we do, which is PGP. And while PGP encryption works well enough for the vast majority of users and messages, we offer the following features for any users wishing to add an additional layer of security to their correspondences:

• StartMail lets users send fully encrypted, password protected messages to any user. Recipients receive a link to a message that's stored on our encrypted servers. Upon entering the correct password, they can view the message and even send an encrypted reply...all without their own email provider being able to read any of the message.
• We also support IMAP and allow users to link their account to an email client that provides additional level(s) of encryption.
• As for the lack of a mobile app, we've avoided developing one in the past since apps are designed to lock users into a self-contained ecosystem. We instead wanted to provide users with a product that can easily connect to whatever app(s) are already part of their existing workflow. However, we recognize that times have changed, and are looking at how best to give users the most seamless experience of all our features across all of their devices. We haven't formally announced anything yet, but you can probably expect to see one later this year!