'Proof", like the "proof" against Trump colluding with Russia, or actual proof?
I've watched and read Comey's "I'm not recommending prosecuting her" speech (you can find it at FBI.gov), and it clearly and specifically says that she broke the law, knew or should have known she broke the law, and others did too. Not sure why, based solely on that speech, that she and many others are not already in jail. That's just what was known and investigated already. Why Sessions isn't doing something about it, I have no idea.
However, she is also known (facts) to have violated her agreement with Obama (and the State Dept) and there's a lot of smoke in terms of her selling the influence of her office. Certainly more than enough to investigate - especially given the minuscule height of the bar set by investigating the Trump campaign colluding with Russia.