Manually Archiving your mail
If you want to remove items from your mailbox but not delete it, setup an archive – (archive.pst). Periodically moving items from your mailbox into an archive frees up space in your mailbox.
However, please keep in mind that once your mail is archived, and thus removed from the Microsoft Exchange Server, ICS can no longer backup those items. If your personal computer crashes, or if the archive file gets corrupt, we cannot restore it. It is therefore important that you backup this archive file every time you archive your email by copying the archive file into another storage device, such as a portable hard drive or a USB flash drive. Also note that archives are only accessible on the computer where the archive file is stored and it is not accessible via Outlook Web Access.
See steps to archive in this document: