Per the Microsoft support policy specified in the following KB, CDO 1.21 is not supported in a .NET Framework environment.
Instead it recommends, to make use one of the following technologies to build the .NET Framework application, the supported options for accessing Exchange data are:
• CDO for Exchange 2000 Server (CDOEX) and ExOLEDB through an interop assembly.
• Windows Management Instrumentation (WMI).
• Collaboration Data Objects for Exchange Management (CDOEXM)
Many of the messaging developers asked us the question, why it’s not supported under the .Net environment. With regards to the issue, please find the detailed article by Mstehle.
Also find the one more related article where many messaging professionals gets confused what does “unsupported” mean? by pcreehan.