You need to select the Reroute incoming SMTP mail option on the Routing tab of the Internet Mail Service Properties sheet. Although this choice might seem counter to the objective, it provides a more secure system if you configure it properly. Afteryou’ve selected this option, you must specify all the domains for which your IMS handles incoming mail. For example, assume that you have no internal or external systems that need to use the Exchange server as a relay and that you’re not using any POP or Internet Message Access Protocol (IMAP) clients. Check out http://support.microsoft.com/support/kb/articles/q1966/26.asp for more info