A service bug that left a group of Windows Live Hotmail users without access to new messages and entire folders for days has been explained and remedied against future instances.

Writing on the Windows Team Blog, Mike Schackwitz of the Hotmail team says the problem stemmed from an error with an automated script that Microsoft uses to test the service for errors in every day usage. Part of the script's function is to clean its tracks once it's done creating test accounts, but this time around the testing jumped the test group and went to real user accounts.

The good news, at least, is that the data is still there. "Please note that the email messages and folders of impacted users were not deleted' only their inbox location in the directory servers was removed," Schackwitz said. The empty mailboxes those who were affected saw when logging in were made to compensate for the fact that their account didn't match up with Hotmail's database. "This is why the accounts received the 'Welcome to Hotmail' message," Schackwitz explained.

That bad news is for anyone who was affected by the bug and didn't log in during the time it was being fixed, Schackwitz said. For those people, any messages sent would bounce back to the senders as if the account was shut down.

The script bug affected 17,355 users--16,035 of which Schackwitz said had their accounts fixed a day after the company first began addressing the issue. The other 1,320 took another three days to get sorted out.

In order to keep a bug like this from happening again, Microsoft is splitting up its service testing accounts from the set of normal user accounts, as well as adding a service status to its support forums and bug reporting tools.