Not only what you mention, but you have the ability to specify where the email auto-archive is stored. Local computers aren't backed up. Networks are, as part of good IT protocol. Any IT pro worth their weight would have made sure the bigwigs' auto-archives were being saved to a location on the network, where they could be part of the regular network back up routine. They do this for just such contingencies as HD crashes. Sorry, IRS, those emails are out there. Not buying the cover story in any form.
You’re absolutely right. The server drives are a contingency against loss of the hard drives. Also, there should be Help Desk logs that captured the nature of the disk failures and there should have been procedures for recovery of such data outlined in a Disaster Recovery Plan.
This story needs to be methodically investigated from the bottom up (i.e., bring in the IT people and their contractors and ask them what happened). Don’t get confused by the smoke screen being put up by corrupt managers.