But email providers don't need end user cooperation to implement intra-provider encryption, i.e. encryption of intermediate hops before it's finally read. (If you email another gmail user, Google can keep the whole voyage encrypted, and likewise if you email a Yahoo user after they agree to encrypt email between the services.)
Yes, such schemes are leaky, as they allow the attacker to read it at any link between providers that haven't cooperated to set up a secure link. And it probably doesn't change the "expectation of privacy" issue, as the provider will be applying encryption to the plaintext, but it decreases the attack surface without requiring end user action.
Yes, such schemes are leaky, as they allow the attacker to read it at any link between providers that haven't cooperated to set up a secure link. And it probably doesn't change the "expectation of privacy" issue, as the provider will be applying encryption to the plaintext, but it decreases the attack surface without requiring end user action.