I have had hard drives crash with irretrievable data lost (with the exception of back-ups).
Still, I never sent a broad distribution e-mail to the effect that my computer had crashed. Only a select few, if any who might have sent e-mails which were downloaded very recently would be notified, if anyone.
The IRS e-mails were legally required to be backed up and preserved.
The documents should have been on the server when she got back up and running with another computer or hard drive, should have had separate back-ups, and should still, if official, have been present on another set of hard drives as well (those of the other computers sending or receiving the e-mails.
Multiple redundancies should have ensured these documents were not lost, as they are legally required to be preserved.
The absence of those e-mails isn't a case of "oops, my drive crashed", but the result of purging them from multiple computer systems.
What I’m wondering is whether the backed up MBX files in fact did contain those emails, but because it’s “too much bother to look for a particular email” they never tried restoring MBX files then scanning them.