I work for a large financial institution. It might be hard for younger folks to believe, but when I started here in 2001, which really isn't so long ago, executives didn't even use email -- they had secretaries to print out their messages. So it doesn't take much for me to believe that, just as smart phones were starting to become a thing, a not-very-tech-savvy, boomer-aged executive took the advice of the first person who came along and said she could keep her BlackBerry. Basically, she wanted a productivity tool for her job, and her employer could not or would not provide it, so she got it set up for herself. That is the beginning and end of this story.
It should be noted: Clinton didn't hide this! She used it openly and with the apparent knowledge (and expressed concern) of all relevant parties. So is it even bending the rules if she's doing it with the knowledge and without the sanction of the parties who oversee electronic security for the State Department? Hell, I don't know. IANAL, YMMV. But I didn't think so, and since the FBI has now confirmed that no laws were broken, at this point I really do not care.
Moreover, yesterday the State Department seemed to contradict Comey's and the FBI's contention that any classified material was sent:
MR KIRBY: Generally speaking, there's a standard process for developing call sheets for the secretary of state. Call sheets are often marked -- it's not untypical at all for them to be marked at the confidential level -- prior to a decision by the secretary that he or she will make that call. Oftentimes, once it is clear that the secretary intends to make a call, the department will then consider the call sheet SBU, sensitive but unclassified, or unclassified altogether, and then mark it appropriately and prepare it for the secretary's use in actually making the call. The classification of a call sheet therefore is not necessarily fixed in time, and staffers in the secretary's office who are involved in preparing and finalizing these call sheets, they understand that. Given this context, it appears the markings in the documents raised in the media report were no longer necessary or appropriate at the time that they were sent as an actual email. So it appears that those --
QUESTION: That the calls were already made?
MR KIRBY: -- no -- that those markings were a human error. They didn't need to be there. Because once the secretary had decided to make the call, the process is then to move the call sheet, to change its markings to unclassified and deliver it to the secretary in a form that he or she can use. And best we can tell on these occasions, the markings – the confidential markings – was simply human error. Because the decision had already been made, they didn't need to be made on the email.(Emphasis mine.)
ETA: here is a good piece that further explains why this case is mostly nonsense.