You need to turn off cached Exchange Mode, but keep in mind Outlook will be slower to process changes in the Global Address List, or will be unable to find certain emails when you search for them. Follow these instructions:
Open Outlook
Click the File tab.
Click Account Settings, and then click Account Settings from the drop-down.
On the E-mail tab, click the Exchange account, and then click Change.
Under Microsoft Exchange server, select or clear the Use Cached Exchange Mode check box.