This warning is sent automatically to inform you that your mailbox is approaching the maximum size limit.
Your mailbox size is currently 79775 KB.
 
Mailbox size limits:
 
	When your mailbox reaches 75000 KB you will receive this message.To check the size of your mailbox:
 
   Right-click the mailbox (Outlook Today), 
   Select Properties and click the Folder Size button.
  This method can be used on individual folders as well.

To make more space available, delete any items that are no longer needed such as Sent Items and Journal entries. 

You must empty the Deleted Items folder after deleting items or the space will not be freed.  

To turn Journaling off,
  click Tools | Options | Journaling Options and de-select all items.

See client Help for more information.