Also, as has been said repeatedly, the US government doesn’t have a binary “classified” or “not classified”. There are many different levels and administrations introduce/adapt them as necessary[1], and there is a practice of retroactive classification.
[1] https://en.wikipedia.org/wiki/Classified_information_in_the_...
Note that Hillary Clinton was not prosecuted despite the subsequent administration basically running on a promise to do so.
Official business with classified information is never done via email, even if everyone is using the government email servers. There are separate networks, devices, and protocols for storing and operating with classified information.