It is end to end if you're using the Protonmail apps

on your phone, there is also a Bridge which allows you to use an application like Thunderbird, Applemail or Outlook but I'm not sure where the decryption occurs there, I assume on the local device though. If you send encrypted mail to someone not using Protonmail it is decrypted in their browser as a single message interface on the Protonmail server.

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