One article said the NSA found the original email during routine scans of government employees. It was the NSA who went to FBI. I have not seen that confirmed, just putting it in the record as a possibility.
There might have been a Florida couple with a Radio Shack wireless network card that inadvertently received his e mail.
NSA grabs everything, almost all of it innocuous garbage intel wise. The computer scanning would have flagged DCIA's communications based on key words, phrases, recipient, and other factors before being brought to the attention of human eyes.
"Say babe, how about we knock some boots tonite". Doesn't strike me as a red security flag with a Drudge Siren.
Real question is was Petraeus under constant surveillance, routine. Scheduled scan, event driven (Benghazi), command driven (O wants to gauge loyalty of flag officers pre-election)
I suspect that the real time monitoring of the Benghazi assault and the stand down orders caused a flurry of encrypted msgs among general staff with an angry WTF tone) The knock down of Petraeus was a message to every flag officer, "Four stars don't mean shit, we own your ass! STFU!"
That makes sense. CIA uses (or at least should, you never can tell these days) highly encrypted email for “all everything”.
The US government regularly uses Advanced Encryption Standard (AES), which is good enough for most traffic, but there is no way the CIA would be satisfied with just that.
Kicking it up enough notches so that the rest of the civilized world can’t break it takes serious effort, however, to the point where only a few, particularly the NSA, would be able to crack it.
But that’s their job. And National Security anything, no matter whose toes are stepped on, is authorized for them. And from there, it’s a pipeline to the FBI, the only agency with enough clout to follow it up.