If the size of your mailbox begins to approach your quota limit, or if you would simply like to clean out your mailbox while maintaining access to the removed items, archiving to your local computer is recommended. Sent Items, for example, is a folder from which you might typically want to archive.
Viewing your quota in Outlook 2010
By default, everyone using the IS&T Exchange server has 1 GB of space. After the current Exchange server is upgraded to Exchange 2010 (scheduled to begin in September, 2011), the default quota will increase to 4GB.
You can view your quota usage at any time in Outlook 2010. Go to the File menu and select Info. You will see a progress bar under Mailbox Cleanup that indicates how much space is left in your mailbox.
Archiving a folder in Outlook 2010
Within that same Mailbox Cleanup area you will see Cleanup Tools and can select Archive. You can archive using either a manual or automated process, although you might want to try the manual option first, until you become comfortable with how archiving works.
Microsoft provides detailed instructions on the archive process for Outlook 2010 at http://office.microsoft.com/en-us/outlook-help/archive-items-manually-HA010355564.aspx
Accessing the Archive in Outlook 2010
After the file is archived, you will be able to access it through Archive Folders which you’ll see listed below your regular mailbox.
Archiving a folder in previous versions of Outlook
Microsoft also provides these instructions for archiving from previous versions of Outlook:
What about Outlook 2011 for the Mac?
Unfortunately, there is no comparable Archive function in Outlook 2011.