Hotmail missing email problem was caused by automated test malfunction
The Hotmail team has since posted an explanation. As part of their regular monitoring and maintenance, Hotmail runs certain automated test scripts -- one of which got a bit carried away. Normally the script manipulates a set of test accounts created by the Hotmail team. This time, however, it accidentally messed with the directory information of up to 17,000 users.
The blog post points out that no email messages or folders were ever actually deleted -- Hotmail simply couldn't associate those users with their inboxes. The problem was protracted because Hotmail's support ticketing system didn't place a high priority on initial reports since there were so few, and because this wasn't a previously-known problem.
To make sure this situation doesn't occur again, Microsoft has tweaked its scripts and the ticketing system. Now, when multiple users report data loss, their tickets will automatically be tagged as high priority and demand immediate attention. Microsoft has also pledged to update its feedback system in order to improve communication with users.