For proof, Twitchy posted this tweet from 2009, in which Sonasoft promoted their relationship with the IRS:
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 company lists the IRS as a customer.
If the IRS uses Sonasoft products to backup their servers why wouldn't you choose them to protect your severs? http://www.sonasoft.com— Sonasoft.com (@Sonasoft) October 9, 2009
But, after six years, the agency apparently ended the contract with Sonasoft soon after investigators started looking into the IRS's alleged targeting of conservative groups.