A default message store is one that client applications can use for general purpose messaging tasks. There are a number of optional features for message store providers that become required if the message store provider is to be used as the default message store. They are:
Implementing the special folders: Inbox, Outbox, and search-results folder.
Providing read and nonread reports.
Allowing incoming and outgoing message submissions.
Allowing creation of messages with arbitrary message classes.
Supporting named and multivalued properties.
Supporting the IMSProvider::SpoolerLogon method, even if the message store provider is tightly coupled with a transport provider.