On a technical basis, sending all of the emails outside the State Department's internal email system puts everything on the public internet, unencrypted, so foreign countries can more easily spy on the USA.
Hillary Clinton Used Personal Email Account at State Dept., Possibly Breaking Rules
Hillary Rodham Clinton exclusively used a personal email account to conduct government business as secretary of state, State Department officials said, and may have violated federal requirements that officials’ correspondence be retained as part of the agency’s record.
Mrs. Clinton did not have a government email address during her four-year tenure at the State Department. Her aides took no actions to have her personal emails preserved on department servers at the time, as required by the Federal Records Act.
