Have a look at this article.  Here is an unintended consequence of Clinton's private server, coupled with a FOIA request.  Deb Riechmann writes:

The names of CIA personnel could have been compromised not only by hackers who may have penetrated Hillary Clinton's private computer server or the State Department system, but also by the release itself of tens of thousands of her emails, security experts say. (snip)

At least 47 of the emails contain the notation "B3 CIA PERS/ORG," which indicates the material referred to CIA personnel or matters related to the agency. And because both Clinton's server and the State Department systems were vulnerable to hacking, the perpetrators could have those original emails, and now the publicly released, redacted versions showing exactly which sections refer to CIA personnel.

"Start with the entirely plausible view that foreign intelligence services discovered and rifled Hillary Clinton's server," said Stewart Baker, a Washington lawyer who spent more than three years as an assistant secretary of the Homeland Security Department and is former legal counsel for the National Security Agency.

If so, those infiltrators would have copies of all her emails with the names not flagged as being linked to the agency.

In the process of publicly releasing the emails, however, classification experts seem to have inadvertently provided a key to anyone who has the originals. By redacting names associated with the CIA and using the "B3 CIA PERS/ORG" exemption as the reason, "Presto - the CIA names just fall off the page," Baker said.

So the State Department releases redacted emails with codes delineating the reason for the redaction.  Any hacker who has the original emails now has a "key" to ID U.S. intelligence assets (spies).