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For emails between Proton Mail users, it's end-to-end encrypted, meaning the encryption happens client side before the message even reaches the Proton server. If a message is coming from Gmail, then of course that cannot be encrypted until it reaches the server, unless of course the Gmail user is using PGP, which Proton Mail is interoperable with.

How do you suggest they technically not read the email at all? It is delivered unencrypted by Gmail, what do you want protonmail to do with it?

Well they double pinky promise, of course