You publish the Custom Form provided by SAP in the Microsoft Exchange server.
The administrator should have rights to publish a form in the Organizational Forms library. For more information refer to http://support.microsoft.com/default.aspx?scid=kb;en-us;271816 .
The components can be found at <irj>\root\portalapps\com.sap.netweaver.coll.appl.gw\WEB-INF\external\davxchg
You notice that the Message class is changed to IPM.Appointment.SAP. This is your Custom Form name which should be specified in the WebDAV Transport configuration.