About: CDO 1.21

API Interface: Mailbox Accessor
API Interface: COM Library with no custom interfaces (IDispatch only)

Business Logic: Messages, AddressBook, and default mailbox Calendar.
Language Support: Any *unmanaged* COM compatible language (C++, VB6, VB Script).
Direct Property Access: Yes.
Product Versions: All versions of Outlook and Exchange up to 2003.  Also available for download.

CDO 1.21 (also known as CDO.dll or CDO 1.2.1) is a COM-based (IDispatch only) API based off of Extended MAPI which provides business logic for calendaring objects and address book information.  It has been around for quite a while and has been available through Outlook (version with security prompts) and Exchange (no security prompts).  CDO 1.21 does NOT ship with Outlook 2007 or Exchange 2007.
Points of Interest:
  • CDO 1.21 is built on top of Extended MAPI.
  • CDO 1.21 is not supported in a managed process.  So, don't use it in.NET code.
  • CDO 1.21 is not supported in COM+.
  • Extended MAPI and in turn CDO 1.21 has different limitations on Exchange and Outlook
  • CDO 1.21 is installed as a part of Outlook 2007 and prior  and also Exchange 2007 and prior.
  • CDO 1.21 is no longer supported for use with Outlook 2010 and later.
Overview of CDO

Collaboration Data Objects, version 1.2.1
Beginning in Exchange Server 2007 and Outlook 2007, CDO 1.2.1 will no longer be provided as a part of the install of the product.  As a result, there is functionality missing that many applications depend upon.  CDO 1.2.1 is a package providing access to Outlook-compatible objects through a COM-based API.

How Outlook, CDO, MAPI, and Providers Work Together

Collaboration Data Objects (CDO) 1.2.1 is not supported with Outlook 2010 and later versions

FYI: Why are MAPI and CDO 1.21 not supported in managed (.NET) code?

OUTBOX: Understanding the Exchange store and CDO 1.21

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