Where can I learn more about changing the total Exchange Server 2003 mailbox limits?
http://www.microsoft.com/exchange/evaluation/sp2/topten.mspx In order to prevent the database from growing unexpectedly after an upgrade, the limit is set to 18 GB and can be set up to 75 GB by using a registry key. For more information about setting the limit, see the Database Size Limit Configuration and Management on the Microsoft Web site (http://www.microsoft.com/technet/prodtechnol/exchange/guides/E2k3TechRef/6c4ce7bb-5289-4221-a87c-b9c78ab7c09c.mspx).