All preprocessors must be registered by transport providers. The transport provider for a preprocessor registers it along with its other logon processing at the time that the MAPI spooler logs on to the transport provider. To register a preprocessor, the transport provider calls its support object's IMAPISupport::RegisterPreprocessor function. It passes in the name of the DLL in which the preprocessor is implemented and the names of the functions in that DLL that implement the preprocessor.
The transport provider can also control what specific addresses or address types the preprocessor applies to. The preprocessor can apply to specific addresses by giving a specific MAPIUID structure to the RegisterPreprocessor method. The preprocessor can apply to specific address types by giving the string representation of the address type, for example SMTP. A preprocessor can apply to all address types by giving a null or empty string as the address type. The RegisterPreprocessor method must be called once for each MAPIUID or address type.
For more information, see IMAPISupport::RegisterPreprocessor.