CDOSYS

CDOSYS is a library (component) provided as a part of IIS starting with Microsoft Windows 2000 Server and later. CDOSYS replaces the older CDONTS and should be used for all new installations when possible as CDONTS is not available starting with Windows 2003 Server and later.

CDOSYS has enhanced functionality over what was available in the older CDONTS library. But for the purposes of sending outbound email, they are extremely close to being equivalent to each other.