1) Click Start
2) Click Run and then type regedt, click OK
3) Locate the following key in the registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Exchange Server Name>\Private-<long hexadecimal string>
4) On the Edit menu click add value, and then type the following in the Value name box: Temporary DB Size Limit Extension
5) Select REG_DWORD for the data type, and then click OK.
6) Enter a value of 1, and then click OK.
It’s recommended that you change back to 0 after you have managed to reduce the size of your database.