Serious question. What does a hard drive have to do with email? I’ve accessed my old emails from a tablet in Costa Rica. Unless my computer turns itself on when I’m overseas, I don’t think a hard drive has shinola to do with email.
It depends on your e-mail service and client.
I have used Outlook as my e-mail client since the late 1990s.
Back when I accessed my e-mail through a dial up ISP and a POP e-mail service, e-mails were saved to .pst files on each users' hard drive. Every night, I copied the .pst files to my server (which I regularly backed up).
About 7 or 8 years ago, I upgraded to my own Outlook exchange server. All of the e-mails were saved on my server (which I regularly backed up).
About 3 years ago it was time to replace my exchange server and I found that hosted exchange servers had become much more reliable and cheaper than owning my own. You can get a hosted exchange server account at MicroSoft that includes all of the MS Office programs (Outlook, Word, Excel, etc.) for $20 per user per month. Copies of all e-mails are saved to .ost files on each user's hard drive (which I again copy to my server and then back up). E-mails are also saved on the hosted exchange server, but only 10GB per user under my account.
I think it is almost certain that the IRS has its own exchange servers, which they should be backing up. Copies of e-mails may also be saved to .ost files on each user's hard drive, which should also be backed up.