Message objects can have properties in them that are not in the set of properties defined by MAPI. Such properties can be unnamed or named. Unnamed properties must reside in a range of property identifiers defined by MAPI. Named custom properties reside in a different range of property identifiers defined by MAPI. They are typically used by custom message types. Your message store provider must support named properties if it is to be used as the default message store. Supporting named properties means implementing the IMAPIProp::GetNamesFromIDs and IMAPIProp::GetIDsFromNames methods, and implementing one or more mapping signatures that identify what names go with what property identifiers. For more information, see Defining New Properties and Supporting Named Properties.
Most message store providers that support named properties use a single mapping signature for all objects in the message store. This has two benefits. One is that it is simpler to implement mapping signatures if there is only one to keep track of. The second benefit is that if all objects in the message store use the same mapping signature, then client applications are assured that all property identifiers on messages in the message store actually refer to the same named property. This allows client applications to display columns for named properties in their folder view interface.