This is reported by the New York Times. These 19 emails, between Clinton and Army General David Petraeus, discuss personnel matters and are part of an email chain that started on a different email account before her tenure as secretary of state, but continued onto her private server in late January 2009 after she had taken office. The existence of these emails also calls into question Clinton’s previous statement that she didn’t use the server before March 18, 2009. Clinton and her spokespeople have declined to comment about the discrepancies. (The New York Times, 9/25/2015)
It will later be alleged that about 1,000 emails between Clinton and Petraeus were found in August 2015. However, none of these have ever been made public, including the 19 emails reported by the New York Times.