Not that guy

the Mailstore on Exchange Server 2003 Service Pack 2 (SP2). is limited by default to 18gb but this is easily changed by adding/changing a reg entry and then restarting the Information Store

Taken from http://support.microsoft.com/kb/912375

Important Before you increase the maximum size of an Exchange database,
verify that sufficient hard disk space is available for the larger database.

1. On the computer that is running Exchange 2003 SP2, click Start, click Run, type regedit, and then click OK.
2. Click one of the following registry subkeys, as appropriate for the store
   that you want to increase:
   • For a mailbox store, click the following registry subkey:
     HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server
     name\Private-Mailbox Store GUID
   • For a public folder store, click the following registry subkey:
     HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server
     name\Public-Public Store GUID
3. On the Edit menu, point to New, and then click DWORD
   Value.
4. In the New Value #1box, type Database Size Limit in Gb, and then press ENTER.
5. Right-click Database Size Limit in Gb, and
   then click Modify.
6. Click Decimal, and then type an integer from
   1 to 75 in the Value data box.