You would need to make the Exchange mailbox the default delivery location by following these steps:
1. Choose Tools | E-mail Accounts, then View or change existing email accounts.
2. Under "Deliver new e-mail to the following location," choose the mailbox.

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Sue Mosher, Outlook MVP
Author of Microsoft Outlook 2007 Programming:
Jumpstart for Power Users and Administrators
http://www.outlookcode.com/article.aspx?id=54
> Outlook 2003
>
>> > If a default calendar has been changed to one in a personal folder, how can
>> > the default be changed back to the original calendar in the user's mailbox?