This is exactly it. For Petraeus, even a secure, encrypted email solution wouldn't have solved the problem. Because he would have been seen to have been using it. Even something that encrypted the emails in-transit would have been a problem, because then the FBI still would have discovered a large number of emails b/w him and the mistress. Even if she was a biographer of his, the volume of email (and timestamps) would probably have been a good clue that there was something else going on.