You have a MOSS 2007 server with 2010 October Cumulative Update (CU) or later installed (or Service Pack 3 or later)
Or you have SharePoint Server 2010 with 2010 December CU or later installed (or Service Pack 1 or later)
You have a document library with documents created by UserA
UserA has been deleted from active directory after UserA has left the company but UserA has not been removed from the SharePoint site collection yet
UserB is copying or moving the document created by UserA using the Manage Content and Structure functionality to a different document library.
After this operation, the Author and Editor of the document changes to UserB.
Furthermore UserA will be marked deleted from the site collection
Before 2010 October CU for MOSS 2007 and 2010 December CU for SharePoint Server 2010, UserA remained as Author and Editor of moved/copied documents in the scenario described above.
The SharePoint product group has confirmed that this is an expected change in the product.
The Manage Content and Structure tool makes use of the Content Migration API. The behavior observed prior to the October 2010/December 2010 Cumulative Update was a consequence of a product defect in this underlying API that impacted SharePoint export/import: disabled accounts were incorrectly enabled and enabled accounts were incorrectly disabled; additionally, permissions groups at the destination of import did not accurately reflect the source described in http://support.microsoft.com/kb/2412258.
Instead of deleting users from active directory, keep the accounts disabled to preserve the Author and Editor information in this scenario.