Here is yet another tip for the "could not access CDO.Message" exception.
It turns out that if you install any of the Exchange stuff, it installs the CDOEX.dll. Well, apparently, this dll is now used for all of the CDO.Message stuff from the COM side of things, and overwrites the CDOSYS.dll. Unfortunately, .NET is bound to the CDOSYS.dll, so it blows up when you try to send a message. The trick is to unregister the CDOEX.dll and register the CDOSYS.dll using regver32.exe.

Here is a translated article discussing this in detail.
http://babelfish.altavista.com/babelfish/urltrurl?lp=zh_en&url=http%3A%2F%2Fwww.microsoft.com%2FChina%2FCommunity%2Fprogram%2Foriginalarticles%2FTechDoc%2Fsendmail.mspx

Remember, always use caution when mucking with the registry.

Cheers!
Dave