Add appointment to calendar through Exchange programatically

How can I add an item to a users calendar programatically through the Exchange 2003 Information Store. I need to know the interface or connector to research. I have full Exchange rights and want to be able to automate assigning appointments without the end users interaction or login. I know this is possible as I have a 3rd party enterprise app that does this very thing.

Any guidance would be very much appreciated.

From stackoverflow

• There are CDO (Collaboration Data Objects) and Exchange Web Services

  Jesse Weigert : Use CDO for Exchange 2003 and EWS for 2007/2010. Btw, EWS is MUCH easier than CDO.