[How-Do] - I store mail offline if my Exchange mailbox gets full?

• Outlook has a new feature, which many Exchange users have wanted for a long time: the ability to hide “On My Computer”. 
• By default, if the first and only account you create in Outlook is an Exchange account then this new feature is enabled. 
• You can disable it by going to Outlook menu –> Preferences… –> Personal Settings –> General and deselecting the option Hide On My Computer folders. 

Source: Office for Mac2011 Help