Jeb Bush used his private email account to discuss sensitive topics, including security and military issues, while serving as Florida governor, The Washington Post reported.
The emails, which the Post uncovered during a review of publicly released records, covered Florida National Guard troop deployments after the September 11, 2001 attacks.
But Bush aides told the newspaper that none of the messages contained sensitive or classified information, with many of the referenced events reported in the media.
Bush, a son and brother of former Republican presidents, is weighing a White House run of his own, and has pilloried likely Democratic frontrunner Hillary Clinton for her "baffling" use of a private email account while serving as secretary of state.
Private email systems can more easily be accessed by hackers, thus allowing sensitive information such as troop movements to be exploited by hostile parties, security experts say.
But Bush's camp argues his server was secure since it was kept at his office in Tallahassee until his second term ended in 2007.
Bush aides have indicated that an unknown numbers of the emails using his jeb@jeb.org account had been redacted or withheld from public release because they discuss sensitive security matters.
Bush generally would only occasionally write about sensitive National Guard concerns in emails that "wouldn't contain information that should not be in the public domain," his communications director Tim Miller told the Post.
The newspaper said Bush would also use his personal email account to lure new businesses to the state and discuss military issues.
Under Florida records laws, he later released about 280,000 emails for state archives, accounting for about half the emails on the private server.
In February, Bush posted emails on a website he launched, JebBushEmails.com.
"They're all here so you can read them and make up your own mind," he wrote.