I don't think people want forward secrecy for their email. If they get a new computer, they probably want all their mail on there, right? Isn't porting over their email efficiently at odds with forward secrecy? Also, is forward secrecy compatible with any kind of encrypted search (I know most encrypted search schemes leak too much these days, but if the alternative is not encrypting email at all...)?
Also, how would it work with multiple people in a thread that can be added/removed arbitrarily, or email addresses that resolve to multiple users? Messaging and email seem like different models to begin with.
Also, how would it work with multiple people in a thread that can be added/removed arbitrarily, or email addresses that resolve to multiple users? Messaging and email seem like different models to begin with.