Here's the thing. Most people don't keep e-mails that are more than two years old. At my last job, I purged my e-mail of any e-mails older than 1 year. I think that's what most people do.
As an IT Professional for 30 years I can tell you, you'd be wrong, most people keep every scrap of email they've ever received unless forced to do otherwise, I wish most people would keep it down to a years worth but it just isn't the case. We're Americans and most of us are pack rats ......
As an IT Professional for a similar time frame, I can second that. I see pack-rat mentality at work every day, and it runs into the terabytes for clinicians and administrators.
Add to that, both statutory and policy mandates, regarding telecommunications and record retention and data life cycles and security and chains of custody, and you've got a recipe for a situation where even the most negligent or cavalier or furtive bureaucrat is holding onto every scrap that he-or-she has ever generated.
And, if you're a service bureau, accepting custody of a server - physical or virtual - or its contents - related to a high-level government appointee or department head - one who deals in secret, confidential or similar public-trust -caliber exchanges - you're going to take an image-snapshot of that server the very moment it arrives, on removable media, and you're going to park that in a vault somewhere.
You might very well do a thousand backups of the thing, and tweak and pare-down its content over time, to reduce the size of ongoing backups, but you're always going to have that coming-in-the-door (Handoff Day) snapshot sitting in a vault someplace, as a final line of defense, for archive restoration.
My money is on the Sacred Cow (Handoff Day) Backup existing offline in a vault somewhere under the service bureau's control, and my money is also on the FBI et al, already having that data in their possession.
It takes a minute or two to wade through so much data, so, I'm not surprised that we haven't heard the outcome of that Federal investigation yet, regarding data content.
Teflon Bubba and Teflon Donna can't keep dodging (metaphorical, legal) bullets forever.
The Feds will tell us soon enough, whether Madame Bullshit Artist has managed to dodge yet another, or whether this one finds its mark.