I think there is no built in mechanism for backing up a single mailbox in Exchange 2003.
Have a look at the following articles, maybe they can help you.
<a href=”http://support.microsoft.com/kb/823176″>How to recover or to restore a single mailbox in Exchange Server 2003</a>
<a href=”http://technet.microsoft.com/en-us/library/cc164324(EXCHG.65).aspx”>Back Up Mailbox and Public Folder Databases</a>
Hope this helps.