If this still doesn't solve your problem, then it may also be possible that the user's mailbox is hidden from the Exchange Server address list. To check this, open Active Directory Users and Computers, select the Exchange Advanced tab on the particular user's properties sheet, and verify that the "Hide From Exchange Address Lists" checkbox is not selected.
If Microsoft Outlook still does not authenticate the account name, then try accessing the mailbox using Outlook Web Access (OWA) or from a different workstation. Can you log on? This may provide further insight into whether the problem is workstation- or server-based.
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