Is that a smoking gun  I smell?  It turns out the IRS contracted with a company that provides email backup services starting in 2005.  This first came to light in the Twitter feed of moregenr, who noticed that the IRS appears on the client list of email  archiving service provider Sonasoft.

The IRS is listed as a customer of Sonasoft software - "Email Archiving Done Right" http://t.co/DoRYpk6n2R

— Morgen (@morgenr) June 20, 2014

This was was picked up by Peter Suderman of Reason, who writes:

The IRS had a contract with email backup service vendor Sonasoft starting in 2005,according to FedSpending.org, which lists the contract as being for "automatic data processing services." Sonasoft's motto is "email archiving done right," and the companylists the IRS as a customer.

In 2009, Sonasoft even sent out a Tweet advertising its work for the IRS. 

The exact details of the service that Sonasoft provided to the IRS aren't clear. But the company advertises its email archiving solution as "ideal for small and medium businesses, government agencies, school districts, nonprofit organizations using Microsoft’s Exchange Server." And a document posted on its website describing its services says that its system "archives all email content and so reduces the risk of non-compliance with legal, regulatory and other obligations to preserve critical business content." 

Hat tip: Clarice Feldman