Are lost IRS e-mails “unbelievable”? Not really
Found on Ars Technica on Wednesday, 25 June 2014

During a hearing held yesterday by the House Oversight Committee, Committee Chairman Darrel Issa said that it was “unbelievable” that the IRS had lost the e-mails of former IRS official Lois Lerner.
To keep mailboxes within size limits, employees were told to archive e-mails locally in Outlook .PST mailbox files—including messages that they thought met the definition of “official correspondence” for retention purposes.
Maybe the IRS should ask the NSA how it's done.