http://www.thehour.com/news/clinton-email-probe-expands-to-norwalk-s-datto/article_254e32b2-6d58-11e5-80bd-fbe4822df370.html
From a purely non political technical standpoint.
1) If datto was engaged at any point when the emails in question were still on the server, the intial 'image' would contain them. Then hourly (typically) the system will 'snapshot' deltas of the server. If say 12 months later somebody delete 30,000 emails, the snapshot after the deletion couldn't be mounted to find the emails, but any snapshots made before the deletion would have them all.
2) Datto offers 2 retention policies. The one in effect when they started using datto stores data forever and only truncates it based on user defined settings. If you exceed your allowed storage you are charged monthly fees per extra TB stored. The other option is time based which is 1 year, 3 years and 7 years. Most take the 1 year. If they switched to this option later, than it is quite possible that no snapshots were retained that contained the data if the emails were deleted more than 12 months prior to datto handing over the ""node"" which is the hardware in their data center that receives the backup data from your datto/eversafe device.
3) The vast majority of IT firms and users, who are are not on time based usually don't delete the very first backup 'just in case'.
I'd say odds are better than 50/50 the data is still intact. They also include a utility from On-Track systems that if this was an Exchange server, lets you rapidly search and manage the emails for legal discovery purposes :)