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Which solves nothing if the other party's email is provided by Google. Google is still getting your chats.

XMPP and Matrix other people can host their servers on Google as well.

Fundamentally, if you don't trust the decisions of the person you are talking with, you can't guarantee the privacy of your chats.

You're forgetting E2E encryption.

Delta.chat supports E2E with other Delta users and Autocrypt email clients, so the point stands.