I do have direct and extensive experience with government email systems. Caveat, mostly with other agencies but a pretty wide variety of them. Post 9/11 there were broad government initiatives to ensure systems like this were definitely backed up, backups stored offsite and usually along with some type of COOP initiative on top of all that. Maybe the IRS did not participate in that initiative, but I highly doubt it.

That they "lost" this data is highly unlikely in my book.