Clinton, when she was Secretary of State, decided she did not want to use the standard email facilities used by everyone else in the Government – but would use her own. And not only that – would keep their operation a secret.
Why she made this decision, she has never said. But whatever the reason, they were a PR disaster. It looked like she was trying to hide some information – and maybe she was.
If she wanted to encrypt her emails she could have done this easily enough – using an app like Virtru, which I am using myself.
Correct me, if I am wrong about this.