The Information Store limit within Exchange 2003 Standard Service Pack 2 has been increased from 16GB to 18GB. But the limit can also be increased to upto 75GB by adding a registry key.
Microsoft Exchange Server 2003 Enterprise Edition has a database size limit of up to 8TB so you may want to look into upgrading to the enterprise version or a later version of Exchange.
Please be careful when making any registry changes, if unsure please seek help. Always make sure you have a full backup before making any changes. Also make a backup of the registry.
To add the registry key to increase the database limit follow the instructions below carefully:
1) On the Exchange 2003 (SP2) server, click start, click run, type regedit and then click OK.
2) Click the following registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Private-Mailbox Store GUID
3) On the edit menu, click new, and then click DWORD Value.
4) In the new value box, type Database Size Limit in Gb, and then press enter.
5) Right click the key Database Size Limit in Gb and then click modify.
6) Click decimal, and then type 1 to 75 in the value data box. Each number equates to GB, so 50 would be 50GB
7) Click OK and exit
8.) Restart the Microsoft Exchange Information Store service.
9) Examine the Application log with in the event log to check that the database size has been set. Event ID is 1216.
For futher info please see article KB912375
The above fix can be used if your information store is not mounting due to reaching its 18GB limit
If you’re running Exchange 2003 Standard SP1 you may want to update to Service Pack 2. But SP1 will allow you to temporarily increase your information store limit from 16GB to 17GB by adding a registry key. Exchange 2000 will also allow you temporarily increase the limit by 1GB. See Microsoft Article: kb828070 and kb813051