I've been in the computer security industry for decades, 20 years ago I did intrusion detection and computer network security for a large government location as part of a team of about 12 people. I referred people to the Inspector General for prosecution when they attacked our networks.
I am really shocked at the level of incompetence here. Even 20 years ago we had 300,000 to 450,000 probes and hack attempts on that one network alone, on a weekly basis. It seems almost certain that her emails were compromised.
Destroying them is inexcusable, at the very least those records could have been used to determine what data had been potentially compromised.
I can't imagine what went on at the State Department where this kind of thing could happen. And, it isn't just Hillary Clinton who is guilty here.
I remember the first day after my security clearance and background checks completed, they spent several hours explaining security policies to me; that felony charges would be brought against me if I walked out the gate with so much as a hard copy of an internal email. I only had the lowest level of clearance for "classified" of "confidential" and even then only as was relevant to performing my job. There were entire buildings in our complex that had "secret" data, and only certain members of our team could investigate those systems for potential intrusions.
Even at that level, when you work with potentially classified material, the default behavior is to act is if all information is classified. If you want to published, or release information, it needs to checked and approved, even if it's not sensitive, just to be sure.
The only excuse I can possibly imagine, is that the clintonemail.com server was actually an elaborate ruse to leak misinformation to foreign actors.