Cause: There is a problem with the Spotlight index for your Outlook items.
Solution: Rebuild the Spotlight index for Outlook
Outlook utilizes the Spotlight feature of Mac OS X to search and filter Outlook items. If searching and filtering doesn't work as expected, you can recreate the index file that Spotlight uses to find items.
On the Apple menu, click System Preferences.
Under Personal, click Spotlight.
Click Privacy, and then click Add .
Navigate to /Users/ username/Documents/Microsoft User Data/Office 2011 Identities/, and then click Choose.
The Office 2011 Identities folder is added to the Privacy list.
Select Office 2011 Identities, and then click Remove .