To install the Internet Mail Service Sample Extension DLL
From the Exchange\Samples\IMCext directory on the Microsoft BackOffice Resource Kit, Part Two compact disc, copy Imcext.dll to a server running the Internet Mail Service.
Create a registry value named ExtensionDLL that points to Imcext.dll.
The key is HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet
\Services\MSExchangeIMC\Parameters. The type is REG_SZ. The value is c:\path\Imcext.dll, where path is the path to the sample extension DLL.
In the Imcdata directory on the Microsoft Exchange Server computer, create a configuration file named Imcroute.cfg.