Transport providers, message store providers, and gateways call the ITnef::SetProps method to set properties to include in the encapsulation of a message or an attachment without modifying the original message or attachment. Any properties set with this call override existing properties in the encapsulated message.
SetProps is supported only for TNEF objects that are opened with the TNEF_ENCODE flag for the OpenTnefStream or OpenTnefStreamEx function. Any number of properties can be set with this call.
No actual TNEF encoding for SetProps happens until after the ITnef::Finish method is called. This functionality means that pointers passed into SetProps must remain valid until after the call to Finish is made. At that point, all objects and data passed into SetProps calls can be released or freed.