With Outlook open, RIGHT click on your mailbox and LEFT click on Data File Properties.
Click on Folder Size
You will see a list of folders and their corresponding size. These are the items that are going against your quota. To reduce the size of your Mailbox, delete items you no longer need or move them to a personal folder (See How to Create Personal Folders). Look specifically at the sent items and deleted items folders. You can empty your deleted items, by right clicking on the deleted items folder and choose empty.