The US SoS has to communicate with lots of people. They, she and those others, need to discuss much that is highly classified. It’s in the nature of the job. Since Clinton as SoS was only reachable through clintonemail.com it stands to reason that everyone who had to communicate classified material to the SoS had to do so through the unsecured clintonemail.com server. I would expect that there are hundreds of instances like this and that there are many in Washington sweating bullets about revelations yet to come regarding their violations of national security laws.
That doesn’t explain why Clinton, a classifying authority, had Mills and other remove classifications before they gave them to Clinton to email. By the nature of the job it is incumbent of the SoS to recognize and protect classified material - she IS a classifying authority remember. It is also true that having such material - whether marked or no - is no excuse for not recognizing the materiel as sensitive and classified, like for instance (SAP) the Order of Battle in the event the US and Israel invaded Iran, or the disposition and IMAGES of North Korean missile batteries.
There is no excuse or justification, no matter how tortured, which exonerates or in any way mitigates her.