The previously undisclosed February 2009 emails between Clinton from her then-chief of staff, Cheryl Mills, raise new questions about the scope of emails from Clinton's early days in office that were not handed over to the State Department for recordkeeping and may have been lost entirely.

Clinton's presidential campaign has previously claimed that the former top diplomat did not use her personal "clintonemail.com" account before March 2009, weeks after she was sworn in as secretary of State.

But on Thursday, the watchdog group Judicial Watch released one message from Feb. 13, 2009, in which Mills communicated with Clinton on the account to discuss the National Security Agency's (NSA) efforts to produce a secure BlackBerry device for her to use as secretary of State.  

The discovery is likely to renew questions about Clinton's narrative about her use of the private email server, which has come under scrutiny.