Outlook 2010 : Why CDO 1.2.1 not supported with Outlook 2010?
Collaboration Data Objects (CDO) 1.2.1 is a client library that provides a thin wrapper over Extended MAPI functionality. CDO does not install with Outlook 2010, and is available only as a download. For more information, see Collaboration Data Objects, version 1.2.1 in the Microsoft Download Center. CDO is not guaranteed to work correctly in an environment with multiple Exchange accounts. CDO 1.2.1 is a 32-bit client library and will not operate with 64-bit Outlook 2010. Therefore, CDO 1.2.1 is not supported for use with Outlook 2010. Most of the CDO 1.2.1 functionality has been incorporated into the Outlook 2010 object model.
Then, what’s the alternative? As an alternative to CDO 1.2.1, update existing solutions that depend on CDO to use the Outlook object model or to use MAPI directly.