Avoiding Unsupported Technologies in Managed Outlook Add-ins
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.
Nonetheless, Microsoft Outlook offers 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), without having to resort to CDO. For more information about a more comprehensive Outlook platform introduced in Outlook 2007 to reduce reliance on CDO and ECE, see What's New for Developers in Outlook 2007 (Part 1 of 2).