RE: exchange, etc. If they are relying on the email server to do their document retention then they are doing it wrong. Heck, even if they were just dealing with legal issues like discovery requests that would be the wrong answer. Any organization that has to deal with litigation (pretty much everyone these days) needs an effective way to deal with email discovery request.
I pulled PST copies from the server 15 years ago for discovery requests and spent weeks going through the files looking for relevant information. That was a very expensive proposition - even if you discount the risks of screwing up a discovery request. This is a problem that tech companies solved a long time ago. There have been many document retention / discovery solutions (both on premises and off-site services) for many, many years now.
With the vast volume of document requests the IRS must get I do not believe that they could possibly be making do without a professional archiving/search solution. Please tell me that an organization of nearly 90,000 people that deals entirely in sensitive, private information wasn't relying on local OST files for document retention in 2010. Of course if they were/are, I suppose that explains a large chunk of those 90,000 employees.