Certain technologies that predate the .NET Framework are not supported in managed code programming. These technologies include Collaboration Data Objects (CDO), Messaging Application Programming Interface (MAPI, often known as Extended MAPI), and Simple MAPI. These technologies were designed and developed with unmanaged code, and Microsoft does not provide official managed wrappers to support their use in managed applications. For more information, see the section "API's that are supported in managed code" in the article KB 266353: The support guidelines for client-side messaging development.
If you want to know why the MAPI and CDO 1.21 is not supported in the managed .Net application, please refer Matt Stehle’s blogpost available: http://blogs.msdn.com/mstehle/archive/2007/10/03/fyi-why-are-mapi-and-cdo-1-21-not-supported-in-managed-net-code.aspx.
Nonetheless, Microsoft Office Outlook 2007 has offered many object model features that achieve what previously only CDO and Exchange Client Extensions (ECE) solved for developers. If you use CDO in an existing unmanaged Outlook application and the lack of support for CDO in managed solutions has hindered you from migrating the application to managed code, you can now consider updating your solution to managed code, using only the Outlook object model and the Primary Interop Assembly (PIA) for Outlook 2007, without having to resort to CDO.