Kissinger made transcripts of some of his work-related phone calls. After he left office in January 1977, he took the only copies with him and eventually had them transferred to the Library of Congress, with tight restrictions on who could access them. A watchdog group sued for access, but the US Supreme Court ruled in a five-to-two decision that the State Department had no obligation to search for documents that had been removed, even if they had been improperly taken.
However, there is a footnote written by Justice William Rehnquist that the ruling might not apply when someone is actively trying to thwart the Freedom of Information Act (FOIA).
In two ongoing civil suits, judges have granted discovery to Judicial Watch in part to determine if Clinton or her aides had actively tried to thwart FOIA. That opens the possibility of a judge eventually ordering Clinton to hand over even the emails she deemed personal, if she still has them. (Time, 5/5/2016)