Updating MAPI Properties
Clients and service providers can update a property value by calling:
An object's IMAPIProp::SetProps method to update the value of one or more of an object's properties.
The HrSetOneProp function to update only one property at a time. Use HrSetOneProp only if the target object is local; this function can cause performance degradation when used with remote objects.
The following procedure illustrates how to use SetProps to update the message class, or PR_MESSAGE_CLASS_A (PidTagMessageClass) property, of a message.
To update the message class of a message
Allocate an SPropValue structure for the message class and set its members as appropriate.
Call the message's IMAPIProp::SetProps method to set the new message class.