Paper trail!
Austin McChord of Datto inc, informed Oversight Committee on October 6, 2015 that his company had a back up of all of Hillary Clinton email.
"several months after Hillary Clinton left the State Department, but that didn’t matter to the Datto box. Its purpose was to capture everything on the server from the moment the box was switched on. Platte River technician Paul Combetta set up the server to automatically delete email every 60 days, as he had been told to, and installed the Datto device to provide localized backups in case the server suddenly went down or hackers managed to take root. The Datto box was also configured to delete anything older than 60 days. What he didn’t realize was the local Datto device was beaming a complete snapshot of the Clinton server multiple times a day to Datto’s cloud servers–and those backups were not being deleted every 60 days. In fact, they weren’t being deleted at all."
https://www.fastcompany.com/3065246/how-this-ceo-got-caught-up-in-the-clinton-email-scandal