A handler is any COM object that exposes (implements) the IExchangeEventHandler interface and advertises (is registered under) the COM category CATID_ExchangeEventSink defined as the GUID value {50F45350-D0AF-11d0-9836-00AA006D27AB}.
Other types of handlers could include those written in Microsoft Visual Basic version 5.0 or later, Microsoft J++ 6.0, and Microsoft Visual C++ 5.0 or later. Using Visual Basic or Java is limited, so it is recommended that you create custom agents using C++.
The mechanism for processing folder events with custom handlers is similar to that used with the Microsoft Exchange Scripting Handler, as shown in the following diagram.
