<snip> Older article but with relevant information on the classification process.
"Of the several thousand emails from Hillary Clinton’s private server that have been released to the public, more than 180 have been redacted, meaning they contain information deemed classified.
But State Department officials decided only in the past few months that this information should be classified.
This sort of classification upgrade occasionally happens when new information comes into play that affects the sensitivity of the information.
Because the information was classified after the emails were sent, no one mishandled this information at the time by sending it over Clinton’s private server.
To send classified information electronically, the State Department has a secure, closed system.
So even if Clinton had used a state.gov email address, this would not have been secure enough to transmit classified information. Procedurally, emails would get a label marking them as containing classified information.
.... The emails released so far don’t have labels marking them classified when sent, which supports Clinton’s argument.
"The fact that no emails on her personal server were marked as classified suggests that she generally was doing her classified business on the secure government servers dedicated to that purpose," she said.
...
Government agencies regularly disagree over what should be classified or not, and transparency advocates say the government overclassifies. (We talked about this at length
in a previous article.) It means, though, that the inspectors general’s findings are not definitive proof that Clinton’s server contained classified information...."
Hillary Clinton's emails: classified or not? PolitiFact
