I worked 28 years for the State Department. There are two distinct email systems, one for classified information and one for unclassified. How did the classified information migrate from one system to another. There had to be intent to circumvent the classification system.
Politicians will claim ignorance of the ,eaning of the term “air gap”
I agree: the classified info found on the server should be matched to the original document, and then the original sender should be investigated and asked: "where did you get this information?"
We have seen at least one public case where Confidential information with markings was pasted into an email. Where did the sender get that?
Frankly, I suspect that a LOT of information was copy/pasted into email on the insecure State Dept. system, and sent to Clinton's server. But, most of the time they remembered to remove the classification headers, and only a few slipped through.