There were three emails marked classified. Only one was actually classified. None of them were marked classified in the header, but the body. The news doesn't report whether Hillary sent or received the classified email. The FBI thinks that whoever sent the email didn't notice the markings.
Nobody would go to jail under those circumstances. To top it off, the reporting I read was that the emails had to do with State department phone call talking points that are declassified after the phone call.