One feature included in Outlook 2010 for the end user is a graphical view of exactly how much space your email is taking up in Exchange relative to the mailbox quota. So how is this feature enabled? The mailbox or the mailbox store where the mailbox resides needs to have two quota values set as per http://support.microsoft.com/kb/982997:
When these values are set, Outlook 2010 will reveal your mailbox usage. To see the graph, open Outlook, click the File tab at the top left of Outlook and look for this:
If you have this misconfigured or if the Prohibit Send and Receive Quota is purposely not set, then the end-user will see this:
Once the values are correctly set, then the graph will reappear.