If you are using Outlook 2003, or possibly Outlook 2007, and are
getting a message about your Personal Folder (.pst file) being full,
you can fix it using the details below:

If you have upgraded from a version Office before 2003, the PST files
that are being created are in a Office97-2002 format, and those pst
files have a limit of 2GB in maximum size. However, you can create a
new “Personal Folder” in the new “Personal Folder File” format that
allows you to grow the file to 20GB. You can then import the
messages from the older PST file into this new file. And then you
can setup Outlook to use that new file as the primary storage file
for email and you can store 20GB of email in a single PST. To change
the primary storage, just go to Tools->Email Accounts-> View/Change
Email Account Settings and in the pull down at the bottom pick your
new Personal Storage file. Once you have done that you can close the
old PST file and not show it in Outlook (just right-click on the top
level of it and choose “close ‘old pst name’”.