Christopher Parsons, a Toronto-based cybersecurity expert with the think tank Citizen Lab, explained the security difference between a personal and official government email.
“The core security advantage is that the U.S. government will be attuned to the risk of her communications being deliberately targeted and, as such, would have a chance to maximize protections afforded to her communications,” Parsons said. “Moreover, data sent and received in U.S. government systems could be protected according to the sensitivity of the communications. So when sending classified or secret documents, a higher standard of care could have been provided.”
I would note that I don’t work at a think tank: I work at the University of Toronto, within the Munk School of Global Affairs.