Ahhh, but they were Blackberry devices. This means emails don't just reside on the originating server and the device, but on what is known as BES, Blackberry Enterprise Server. Not only would you have potentially copies of the emails in the hands of Blackberry, but you would have routing logs for emails.
This is the funny thing about all the hubbub about Hillary wiping her server, that is only one link in the chain an email goes through. Example, BES routing diagram.
I'm actually surprised there hasn't been a bigger call for the routing logs from her network, BB, or other sources. They can find where the other end of the emails or any potential archived copies on various networks are located.
Nothing ever is permanently deleted on the Internet.