With exchange 2003 SP2, the database store in Exchange is increase to 75GB, this is not setup automatic.
You have to change something in the registry database.

To set the size of the private mail store

1. Click Start, click Run, type regedit, and then click OK. Regedit starts.
2. Browse to HKEY_LOCAL_MACHINE/ System/ CurrentControlSet/ Services/ MSExchangeIS/ ServerName, where ServerName is the name of your server that is running Windows SBS.
3. To modify the private mail store size, right-click the Private-GUID folder, click New, and then click DWORD Value.
4. Type Database Size Limit in GB, and then press ENTER.
5. Right-click the new entry and then click Modify.
6. In the Base field, click Decimal. In the Value data text box, type a number between 1 and 75. This is the new maximum size of your database in gigabytes.
7. Click OK.
8. Close Regedit.

Next you maybe want to get a warning when the database is almost at the limit, this can be setup this way:
To set the size of the buffer

1. Click Start, click Run, type regedit, and then click OK. Regedit starts.
2. Browse to HKEY_LOCAL_MACHINE/ System/ CurrentControlSet/ Services/ MSExchangeIS/ ServerName, where ServerName is the name of your server that is running Windows SBS.
3. To set a buffer size for the private mail store, right-click the Private-GUID folder, click New, and then click DWORD Value.
4. Type Database Size Buffer in Percentage, and press ENTER.
5. Right-click the new value and then click Modify.
6. In the Base field, click Decimal. In the Value data text box, type a number between 1 and 100. A value of 10, for example, results in warning events being logged when the database has 10 percent capacity remaining.
7. Click OK.
8. Close Regedit.