Interesting. I would think even those messages end up being backup up on the server but in that case that is the only place it would exist and would never be intercepted through normal means.
Were they that sophisticated?
I think they were knowledgeable about how other enemies of America communicate and could have adopted the practice. That way they could have party influence and direction without a compromising trail. An important question to ask is not, “did you send email,” but, “did you compose email?”