Error: ClassFactory cannot supply requested class, error code: -1056749262

During a migration of Exchange 2000 or Exchange 2003 to Exchange 2007, the following error may occur:

Error was found for Username(user mail address) because: Error occurred in the step: Opening source mailbox. Failed to open mailbox with error: ClassFactory cannot supply requested class, error code: -1056749262

In my case, this error occured because the user was disabled in Active Directory.
If you face this same error, try the following:

  1. Unhide the user in the Global Address Book (open the Properties of the User in Active Directory, go to the Exchange Advanced tab, and deselect the “Hide from Exchange Address Lists” checkbox)
  2. Enable the user, in case the user is Disabled. Also check whether the user is Locked out or not (open the Properties of the User in Active Directory and check the Account tab. Both the chekboxes should NOT be checked)

When one or both of the above were the case, wait for 10-30 minutes. Active Directory needs to replicate its changes. Then retry to move the mailbox to the other server.

If you still face the same error after 30 minutes, try to unmount en mount the Information Store. This should be your last chance for survival :)

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