Mishandling charges are very hard to bring if the mishandling only ever resulted in intended and valid targets of secret informaiton receiving said secret informaiton. The key is distinguishing between targets and channels. Compare HRC to Patreaus.
HRC:
Targets = State subordinates who were entitled to receive secret messages from HRC (Secretary of State)
Channel = insecure private server
Patreaus:
Targets = biographer/mistress who had no right to the black book of special operations in Afghanistan
Channel = handed the book of secrets to her physically
HRC's targets were legitimate. No way you can prosecute a mishandling charge on the basis that the channel wasn't very secure. You need some kind of mishandling where the intent was to move secret info to someone who it wouldn't be legal to send it to. HRC's underlings were all valid targets. Paula Broadwell wasn't.