It does not matter if it was encrypted. Using her private email server for government business was illegal. As a lawyer and high government official, she should have been fully aware of this illegality.
Maximum penalty for her allowed under the law.
Then go after her for the inadequate security with classified documents.
(Is Leavenworth co-ed?)
Of course it’s illegal, encrypted or not.
My question is one of stupidity assessment, not legality.
Heck, if she wanted to set him up for some reason, clear-text, or deliberately weak email security is one way to do it.