As a system administrator, I find it peculiar that a government agency has such troubles with maintaining data effectively. Redundancy, backups, disaster recovery; these terms are the commandments of being in large scale IT. Many other professionals agree with me. Nearly all large organizations run Microsoft Exchange for email, which has built in redundancy to prevent data loss of this scale. Looking past the system itself, backups are your lifeline. Not having bulletproof backups and recovery has cost many admins their jobs, so I hope someone got fired.
The IRS audits us, and if we say "oops, we missed two years because we had a computer glitch", we do not get cut a break. The American people now should now audit the IRS to ensure this never happens again.