I am an I.T. professional and have been for 30 years. I will GUARANTEE those emails are recoverable.
The IRS uses what is called an Exchange server. EVERYTHING that uses Outlook is stored in an .OST file on a SERVER. In the case of the IRS it is almost with certainty that server is located within a data center. That server and probably hundreds of others are backed up with multiple backups AND in other locations.
If the hard drive in a users PC crashes, burns, stolen or whatever, you simply plug in a new one, reconfigure Outlook to grab the .OST file from the server and magically all of your email, folders, contact list, calendar, etc,etc,etc, come back to life on the PC.
Now, these servers are configured with RAID drives that mirror each other. If one goes bad the others keep running. All you do is plug in a new drive and it mirrors again. Plus the servers are mirrored as well. It’s called FAULT TOLERANT.
Even if somebody destroyed the actual .OST files and the backups from the servers. The RECIPIENTS still have those emails and...they are on servers as well.
I would have EVERY computer, tablet, smart phone and any other devices seized from Lerner and EVERYBODY in her contact list as a start. Then I would seize the servers and subpoena EVERY person involved in the administration of those servers. Answers would come swiftly if facing jail time.
The IRS is a criminal organization so you have to think like a criminal. Thing is, they always screw up somewhere.