In Exchange 2007 in order to set the external postmaster address, you had to use the Exchange Management Shell using the following command:
Set-TransportServer -externalPostmasterAddress you choose>
However in Exchange 2010 you have the additional option (you can still use the shell if you wish) of setting this option via the GUI.
In order to do so open the Exchange Management Console and navigate to:
· [ Organisation Configuration -> Hub Transport ]
· Choose the “Global Settings” tab
· Using the right hand mouse button – RIGHT click on the “Transport Settings” entry and from the context menu that appears choose “Properties”
· From the dialog box that appears, on the “General” tab type the postmaster address in the “Specify the external postmaster address field”
· Click “ok”
You can also use the Set-TransportServer command above should you wish.