The Post, citing anonymous sources, said the discovery was made by White House officials reviewing emails in response to a public records lawsuit.
When asked about it, Trump said she was unfamiliar with details of the rules, the Post said.
A spokesman for Trump´s attorney confirmed that she did use a private email account before she was informed of the rules, and said that all her government-related emails had been turned over months ago, the newspaper reported.
President Donald Trump has repeatedly pilloried Hillary Clinton, his rival in the 2016 presidential election, for her use of a private email server for government business while she was secretary of state.
Then FBI director James Comey — fired by the president last year — announced that the bureau would reopen an investigation into Clinton´s use of the private email server just 11 days before the 2016 presidential vote, a move that some say may have helped cost her the election.
Trump supporters still chant “Lock her up!” at rallies, mimicking one of Trump´s battle cries during the election campaign, indicating they believe Clinton should be jailed.