The default limit of each database in Exchange 2003 Standard Edition. With Service Pack 2 this default was changed to 18GB. This limit can be increased up to 75GB. Enterprize Edition allows for databases up to 18TB.

The database size is checked at 5:00am every 24 hours by default (this can be changed). Event ID 9689 will be logged in the application log the first time that the database limit has been exceeded. The second time this event is logged, the database will be taken offline. You can re-mount the database but will have 24 hours to fix the issue before the database goes offline again.

To change the database limit, use regedit to navigate to:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Private-Mailbox Store GUID

and create a DWORD called Database Size Limit in Gb

Enter the new limit in GB, between 1 and 75 for Exchange 2003 Standard and between 1 and 8000 for Exchange 2003 Enterprize. Make sure to enter this number in decimal, not hex.

More info: Petri KB and MS Exchange Team Blog