Heard at least one report about the email system that was used and it was basically the same setup as her husband used while he was president. It was installed under orders from the state department.
It was not illegal to use private emails systems (virtually everyone was using the official network as little as possible) both democratic and Republican.
All the law required at that time was that all work related emails were logged onto the official system if they ware not using official channels. The law requiring everyone to only use official email system can't into effect 18 months after she left.