Hillary Clinton's use of a private email server was a national security risk.
1) Executive Order 13526 and 18 U.S.C Sec. 793(f) of the federal code make it unlawful to send or store classified information on personal email.
2) Section 1236.22 of the 2009 National Archives and Records Administration (NARA) requirements states that:
“Agencies that allow employees to send and receive official electronic mail messages using a system not operated by the agency must ensure that Federal records sent or received on such systems are preserved in the appropriate agency record keeping system.”
3) Use of a private server's intent was to defy and avoid the Freedom of Information Act.