That message is quite clear. The mailbox store on your Exchange Server is hitting the default limit of 18GB. If it is allowed to get to 18GB, your stores will dismount and you will not be able to use the Exchange Server.
Fortunately after you have installed Service Pack 2 on your Exchange 2003 Server, you can raise the database size of the Exchange mail stores up to a maximum of 75GB using a registry change. Follow the information in
http://www.msexchange.org/tutorials/Explaining-Database-size-limit-changes-Exchange-2003-Service-Pack-2.html for details on raising the mailbox store maximum size limit, and you will resolve this issue.
-Matt