If you don't trust your send gateway then you are without a doubt hosed anyway, as they can just strip your signature anyway (or, replace it with one linked to a key they generated in your name on the fly). Yes, if your correspondant is super on the ball and notices that you didn't sign/encrypt this specific message, maybe you win. But if I were a bad MITM I'd just put "Sent from my iPhone" at the bottom and there is the plausible explanation.