Additional important information, obtained from another FR thread:
http://freedomoutpost.com/2014/06/outside-email-archiving-company-backed-irs-emails/
In accordance with best practices, every IT shop (even in the government) has a regular daily/weekly/monthly backup schedule to make sure that all changed files are kept for posterity. They typically keep those backups off-site as a precaution against loss should the main IT center suffer a disaster.
I believe the files are retrievable, unless someone has gone to extraordinary lengths to erase them. The congressional committe needs to immediately call in the support contractors who actually provide IT services and run IRS’ facilities. The answers are there if you know what questions to ask.