ôCollaboration Data Objects (CDO) was originally called "OLE Messaging" and later "Active Messaging". CDO is a COM wrapper of the MAPI library and can be called from any development language that supports automation. [ . . . ] CDO is primarily designed for client activities. CDO implements most but not all MAPI functionality, although far more than Simple MAPI.ö

Reference:
http://support.microsoft.com/kb/q200018/