Check Your Folder Size in Outlook 2003
Open Outlook 2003
Right click on the very first folder. Most likely it is labeled with your name.

Click Folder Size

You will notice the Total Size. Everybody is given 100 meg of space on the Exchange Server. When you get near 100000 KB you need to clean out your deleted items, sent items etc. You can also move the email you wish to keep to a Personal Folder.
Move Email to Personal Folders
Highlight the email(s) you wish to move.

Right click and choose Move to Folder
Choose the Folder you wished to be the destination.

Click OK