Like the 22 million emails deleted during the Cheney/Bush administration that were related to the US attorney firing scandal or that the Attorney General, Roberto Gonzalez, used a private server owned by the RNC for official communications:
http://www.pbs.org/weta/washingtonweek/web-video/missing-white-house-emails
Or when the Secretaries of State in the last administration also used non-government servers to send and receive classified materials:
http://mediamatters.org/blog/2016/03/07/state-dept-concludes-past-secretaries-of-state/209044

OCU