Caroline Kennedy, the US ambassador to Japan and daughter of late president John F Kennedy, has been criticised for using private email for official business.
The latest revelations about a high ranking diplomat were given greater resonance in the light of the scandal hanging over former secretary of state Hillary Clinton.
A report by the State Department inspector general into the management of the Tokyo embassy tagged Kennedy with a similar charge to the one dogging the White House candidate.
"Senior embassy staff, including the ambassador, used personal email accounts to send and receive messages containing official business," the report said.
"In addition, (OIG) identified instances where emails labelled 'Sensitive but Unclassified' were sent from, or received by, personal email accounts."
State Department guidelines require staff to use official email accounts - judged more secure - to send government business, which then form part of the historical and legal record of their posts.
During her term leading the department between 2009 and 2013, Clinton departed from this protocol and used a personal server and private account for all email.
She has since handed over around half of the emails from the server to the department so that they can judge whether they should be classified or made public.
But she has come under intense criticism for potentially exposing official business to hackers and allegations that the 30,000 or so mails that she reportedly had wiped from the server could contain compromising material.
Kennedy, a political appointee of President Barack Obama and not a career diplomat, will not face the same level of scrutiny as Clinton, but the breach is further embarrassment for the State Department.
At a press briefing in Washington, department spokesman John Kirby defended the embassy and its illustrious ambassador.
"It is not prohibited to use private email. It is discouraged, obviously," he said.
"And I think I would also add that the mission in Japan is implementing all the recommendations, including the recommendations with respect to email traffic as we speak."