Ex: The NDA cited a few laws, and apparently the one the FBI is most focused on is the gross negligence provision of this lil guy:
https://www.law.cornell.edu/uscode/text/18/793Have to show a few things here though, notably that using her own server was gross negligence (pretty high standard) and that such material was "removed from its proper place of custody or delivered to anyone in violation of his trust, or to be lost, stolen, abstracted, or destroyed". Negligence is at least partially informed by common practice and apparently State sucks at this since it is super common to use personal email servers for work and Powell did the same (though his camp says they never had classified info....but so did State re Hillary, until they actually took a look and started retroactively classifying stuff).
I dunno man. Not an easy case to make and if you're going to throw up an indictment against a political institution you usually want a very strong case. I'd say the smart money is still against an indictment on the email issue UNLESS a genuine bona fide smoking gun emerges.
Another very interesting question is the allegations of favored treatment for the Clinton Foundation and routing organizations from State to the Clinton Foundation. I've read some stuff saying this is rising focus of the investigation.