In all, 1,274 of Clinton's emails have now been deemed too secret for public release. The mounting number of correspondence with classified material has been a lingering problem for Clinton on the presidential campaign, spotlighting her decision to exclusively use a private email account, routed through a personal server installed at her suburban New York home, for work related correspondence while secretary. Clinton has said none were marked classified at the time they were sent. She and State Department officials have characterized the material has having been "upgraded" to a classified level through the review process. It is the responsibility of individual government officials to properly handle classified material, including to avoid including the material in insecure email and to properly mark it as classified.

Clinton turned over the records to the State Department in December 2014, nearly two years after leaving office, in response to a request from the department. State Department officials had realized the agency had gaps in its records after finding it did not have full access to her correspondence while preparing to submit documents to the congressional committee investigating the 2012 attacks on U.S. installations in Benghazi, Libya.

https://www.washingtonpost.com/news/post-politics/wp/2015/12/31/state-department-misses-court-ordered-goal-on-clinton-email-release/

Now you see it. Now you don’t.

This is not a magician’s incantation. It is a description of retroactive classification, a little-known provision of U.S. national security law that allows the government to declassify a document, release it to the public, and then declare it classified later on. Retroactive classification means the government could hand you a document today and prosecute you tomorrow for not giving it back. Retroactive classification can even reach documents that are available in public libraries, on the Internet, or elsewhere in the public domain.

The executive branch has used retroactive classification to startling effect. The Department of Justice, for example, declassified and released a report on National Security Agency (NSA) wiretapping only to declare, years later, that the report was once again classified. The journalist who had received the report was threatened with prosecution if he did not return it. Retroactive classification has also targeted government documents revealing corruption in Iraq, violence in Afghanistan, and mismanagement of the national missile defense program. In each of these cases, the government released a document in an unclassified form through official channels—not through a leak—and then turned around to classify it.

This practice would be troubling enough if it actually removed the document from the public domain. But in the Internet Age, once a document is released to the public, it is often impossible for the government to retrieve it. While retroactive classification does not remove the document from the public domain, where our enemies can access it, retroactive classification does remove the document from the public discourse,prohibiting members of Congress, government auditors, and law-abiding members of the public from openly discussing it.