When >25% of government workers respond that private emails were used at least some of the time, that's an institutional problem.
http://www.govexec.com/insights/state-internal-workplace-com...
https://www.washingtonpost.com/politics/clintons-experience-...
Except we're not talking about "soft guidelines" or simple recommendations but actual laws:
* Executive Order 13526 - https://www.whitehouse.gov/the-press-office/executive-order-...
* 18 U.S. Code § 793 - https://www.law.cornell.edu/uscode/text/18/793
* The 2009 Federal Records Act
* (probably) The Freedom of Information Act
in addition to a classified information non-disclosure agreement:
* https://foia.state.gov/searchapp/DOCUMENTS/HRC_NDAS/1/DOC_0C...