Windows: Archiving Messages to Reduce Mailbox Size
Messages stored on your computer's hard drive do not count toward your total mailbox size. Storing your messages locally is called archiving and is a great way to reduce your mailbox size on the server.
Note: Archived messages are not available when using Outlook Web App (OWA) or a computer other than the one where the messages have been saved. If you replace your computer, remember to move your archived messages to your new machine. Also be sure to regularly back up archived messages, since this is not done automatically.
You can archive files manually or automatically using the AutoArchive function for any folder:
Right-click the folder you want to archive and select Properties from the shortcut menu.
Warning: Be sure you select the specific folder you want to archive, and not your entire mailbox.
Select the AutoArchive tab.
Select which settings work best for you. For example, you may want to archive items older than 5 days and move them to the default archive folder. You can also archive them to a specified folder on your local machine.
For more information on Microsoft Outlook, visit Microsoft's website.