MAPI can be used to access items and folders within public and private stores, as well as the properties stored along with each item. MAPI is used by various industry-standard e-mail clients, such as the Microsoft® Exchange client, all versions of Microsoft Outlook® and Outlook Express, and many versions of proprietary clients, including those sold by QUALCOMM Incorporated (Eudora) and Netscape Communications Corporation. Previous versions of Exchange Server provided stores that were accessed primarily by using MAPI providers.

A request from a MAPI client, such as Outlook, to view a mailbox store or a public store is sent as a remote procedure call (RPC) to the Exchange Server 2003. Because Outlook is a MAPI client, the request must go through MAPI on both the client and the server. The Exchange store processes the request and maps the requested folders or items to the equivalent MAPI properties. The requested information is then sent back through the MAPI layers to Outlook as a RPC.

The following illustration shows the relationship between MAPI clients and the Exchange store:

Exchange Server 2003 continues to provide access to MAPI clients, such as Outlook, through the Exchange store  MAPI message-store provider. MAPI clients can access mailboxes in any private store on the server transparently. MAPI clients can access only one public folder tree per server. This public folder tree is designated as the MAPI public folder tree, and is normally specified as the /public virtual root through HTTP and as the Public Folders top-level folder through the Exchange OLE DB (ExOLEDB) provider. To access data within other public folder trees and associated public stores, you must use the new application programming interfaces (APIs) and protocols, such as the WebDAV protocol, the ExOLEDB provider, or the file system.