As you know, the Messaging API (COM-like API) that provides access to the contents of messaging stores. CDO 1.2.1 (Collaboration Data Objects, version 1.2.1) is a package providing access to Outlook-compatible objects through a COM-based API. Using either CDO or MAPI, a program can connect to a MAPI store, and then perform operations against that store.
Starting from Exchange 2007, neither the Messaging API (MAPI) client libraries nor CDO 1.2.1 are provided as a part of the base product installation. As a result, there is functionality missing that many applications depend on. Microsoft Exchange MAPI and CDO 1.2.1 provide access to these API's.
From Exchange 2007, Microsoft will distribute the MAPI client libraries and CDO 1.2.1 as a Web download.
You can download it from Microsoft Download Center.
Please note: This version is now compatible with Windows Vista and Windows Server 2008. Note also that this web release follows the Microsoft Lifecycle policy of Exchange Server 2003, which goes into Extended Support in April 2009.