Nearly all senior administration officials have personal email accounts that they occasionally use to communicate about work. But the officials said that Mr. Carter emailed with his closest aides about a variety of work-related matters, including speeches, meetings and news media appearances. In one of the emails obtained by The Times, Mr. Carter discussed how he had mistakenly placed a notecard in a “burn bag.” Such bags are typically used to destroy classified information.

The spokesman for Mr. Carter played down the secretary’s use of personal email, saying that he used it primarily to correspond with friends and family.

“Any email related to work received on this personal account, such as an invitation to speak at an event or an administrative issue, is copied or forwarded to his official account so it can be preserved as a federal record as appropriate,” said the spokesman, Mr. Cook.

“Secretary Carter strongly prefers to conduct communications on the phone or in person, and like many of his predecessors rarely uses email for official government business. The secretary does not directly email anyone within the department or the U.S. government except a very small group of senior advisers, usually his chief of staff.”

Mr. Cook declined to answer a question about whether Mr. Carter had violated the Defense Department’s email policies.

Mr. Carter was assigned a government email account when he became defense secretary in February but continued the use of the private account. In contrast to Mr. Cook’s statement, a former aide to Mr. Carter said the defense secretary used the personal account so frequently that members of his staff feared he would be hacked and worried about his not following the rules.

In 2012, the Defense Department adopted a policy that bars all employees regardless of rank or position from relying on personal email to conduct government business. Last year, President Obama signed a law directing federal officials not to send or receive emails on their personal accounts unless they were copied directly into their government accounts or forwarded to a government account within 20 days. A spokesman for Mr. Carter said that he had done this but did not provide any documentation to back that up.