Exchange is responsible for the publication of Free/Busy information when applications use CDO (such as ERM) for processing meeting requests. ERM does not physically push out Free/busy information as Outlook does. Rather it relies on Exchange to poll for updates to Free/Busy information. Please reference the following link for a detailed description of this process:
http://msexchangeteam.com/archive/2004/08/19/217187.aspx.