If the CIA director cannot keep his email secure from FBI snoopers, no one else can either. Your life will have fewer nasty surprises if you never ever write anything in emails or online that you wouldn't post on the bulletin board down at the supermarket. Because the world is full of snoops who will post anything juicy they find on line. And if it's online, they can find it.
I would have thought an old hand like Petraeus would have known this. And if he didn't know, CIA should have told him.