As someone who knows quite a bit about how e-mails are sent and stored; Logan's description is not entirely accurate.
Depending on the security of the information involved and the architecture of the system, the e-mails may have existed on a single server, or the mailbox on a single server. Usually in an enterprise implementation with thousands of addresses a fixed number of addresses will be assigned to each physical (or virtual device). That device may or may not be mirrored or hot-sited for redundancy. This means that a SPOF incident can wipe out an entire mailbox, or series of mailboxes. That leaves backups. The great millstone about the neck of any organization. Backups are only as good as the last confirmed physical restore of the data contained therein. For mail servers, depending on the implementation, this gets even scarrier, as restoration of a previous version of a mailbox often requires recreation of the entire server and all contents -rather than that one mailbox. For this reasons, the restores are not often tested as often as they should be (cough! almost never! cough!)
So this may be deliberate, it may be that with years of budget cuts, the IRS has not been able to invest fully in the maintenance and renewal of their mail architecture.
These are details that will come out in time.
Depending on the security of the information involved and the architecture of the system, the e-mails may have existed on a single server, or the mailbox on a single server. Usually in an enterprise implementation with thousands of addresses a fixed number of addresses will be assigned to each physical (or virtual device). That device may or may not be mirrored or hot-sited for redundancy. This means that a SPOF incident can wipe out an entire mailbox, or series of mailboxes. That leaves backups. The great millstone about the neck of any organization. Backups are only as good as the last confirmed physical restore of the data contained therein. For mail servers, depending on the implementation, this gets even scarrier, as restoration of a previous version of a mailbox often requires recreation of the entire server and all contents -rather than that one mailbox. For this reasons, the restores are not often tested as often as they should be (cough! almost never! cough!)
So this may be deliberate, it may be that with years of budget cuts, the IRS has not been able to invest fully in the maintenance and renewal of their mail architecture.
These are details that will come out in time.