The Outlook 2007 SDK: MAPI Reference is available to download.
The Microsoft Office Outlook 2007 Messaging API (MAPI) Reference provides the messaging architecture for Office Outlook 2007.
The Outlook 2007 MAPI Reference describes how to create Outlook 2007 items by using MAPI that is based on the Component Object Model (COM) rather than the Outlook object model. MAPI for Outlook 2007 provides a set of classes, interfaces, functions, and other data types to facilitate the development of Outlook 2007 messaging applications. Applications and transports use MAPI to manipulate e-mail data, to create e-mail messages and the folders to store them in, and to support notifications of changes to existing MAPI-related data.