That's really interesting. I wonder though if signing would have a negative effect in terms of privacy?

i.e with message signing, a third party can prove that a message was sent by you. Whereas with an unsigned email, you'd at least be able to plausibly deny that you sent it. You could claim it was forged.

But then again, with secret and silent data collection systems you're not really in a position to deny anything anyway. I have a feeling you'd have a hard time denying an unsigned email with your name on it to a court if it came to that.