The IRS is claiming that their exchange server had such a small mailbox quota that she was forced to download her emails to a .PST file which is stored on her local drive and removed from the server.
Without the ability to send an inspection team or law enforcement in to find out what’s what, the IRS can claim anything. The IRS are the ‘foxes in the hen house’ as the old saying goes.
The removal from the server is probably the setting of a flag which takes the email off of her account and schedules the email for deletion...
Once it has been added to a backup file.
If regular users had the ability to delete an email from the server, no emails would ever be saved.