The US state department identified the emails as it carries out its check on whether they can be released publicly.
Mrs Clinton’s use of private email while secretary of state for four years has sparked a barrage of criticism.
The Democratic 2016 frontrunner says her private email contained nothing that was classified at the time.
Critics say that her set-up was unsecure, against government rules and designed to shield her communications from oversight.
Last December, she handed over about 30,000 emails she sent and received while in office.
A team of officials has been checking the emails since a federal judge ordered they be released to the public on a rolling basis over the next five months.
In court papers filed with a US District court judge in Washington on Monday, the State Department updated its progress.
One of its officials told the court that 305 of her emails are being recommended for review by intelligence agencies.
State Department spokesman John Kirby told reporters later that day the review of more emails was “healthy”.
“It doesn’t mean that all 300 are going to end up at some level of (classification upgrade). I suspect some will and I suspect some won’t,” he said.
The FBI is investigating whether classified information was improperly sent via the server and stored there.
Under US federal law, officials’ correspondence is considered to be US government property.