In PSS, one of our most frequent support call generators is when a mailbox store dismounts after reaching the 16GB limit. This 16GB limitation applies to all versions of Exchange Standard Edition. In Exchange 2000/2003, the 16GB limit is the combined size of the .edb and .stm database files.
The good news is that a new functionality has been added to Exchange 2000 and 2003 which will temporarily increase the database size by 1GB to 17GB. This will allow the Mailbox Store to remount, and give the Administrator a window in which to delete unnecessary database content and prepare for offline defragmentation. In Exchange 2000 this functionality is included in the Post-Service Pack 3 Rollup. In Exchange 2003 this is built into the product, however a registry value must be added to enable this function.
For more detailed information on this topic, see the following Microsoft Knowledge Base article:
828070 Exchange Server Mailbox Store Does Not