Not her server their server. People who sent or received e-mails and texts to or from Hillary have hard drives and ISPs.
My point was that she may have assigned emails from her server (there are at least 22 different email addresses found so far originating from her server) and other people logged in to those email addresses and and they exchanged email that way. Since she had complete control over her server, any of those correspondences could have been deleted and not retrievable.
Look up home brew server and you'll see how that is set up and how easy it is to delete every trace of what you sent and received.