Kendall called the request pointless, saying Clinton’s IT staff had confirmed to him the messages are gone for good.

But permanent deletion is extremely difficult to achieve, the experts said. Enterprise servers built in the last decade or so are increasingly designed to preserve emails more rigorously, either as a document trail in case of a lawsuit, to comply with industry regulations or to allow system administrators to “idiot proof” their systems so they can save the day when non-technically proficient executives accidentally delete emails.

The key principle of digital forensics is “Delete doesn’t and restore won’t,” said Mark Rasch, a former federal prosecutor who worked on computer crimes.