Hook providers are MAPI service providers, like message store providers, transport providers, and address book providers. However, hook providers are by far the simplest type of provider to implement. Hook providers are intended to perform sorting and archiving types of tasks with messages. Some typical hook provider tasks are:
Hook providers generally change the relationship between a message and its message store, although they can also change the content of a message.
Hook providers control whether they are called for inbound messages, outbound messages, or both by setting the HOOK_INBOUND and HOOK_OUTBOUND flags in the PR_RESOURCE_FLAGS property of the hook provider's profile section in the MAPISVC.INF file. For more details, see File Format of MAPISVC.INF. In the event that multiple hook providers are being used, the hook providers are called in the order that they are installed into a user's profile.