The Messaging API is a 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 with Exchange 2007, Microsoft will distribute the MAPI client libraries and CDO 1.2.1 as a Web download.
You can download it from here: http://www.microsoft.com/downloads/details.aspx?FamilyID=94274318-27c4-4d8d-9bc5-3e6484286b1f&DisplayLang=en