Just this morning I helped a partner with this very scenario.  Unlike previous versions of Exchange, Exchange 2007 does not provide an interface within its management GUI to specify a custom port when using a SmartHost for outbound mail delivery.  As a result, we need to set this via the Exchange Management Shell.

Once you open the Exchange Management Shell, one simple command will allow you to specify the custom port to use:

set-sendconnector –identity ‘[Send Connector Identity]’ –port [Port Number]

In SBS 2008, the default Send Connector that gets created is named “Windows SBS Internet Send [SERVER]”  where [SERVER] is the netbios name of your SBS server.  So for example, if your SBS box was named  SERVER01 and you needed to use port 2525 to send email to your smart host, you would enter:

set-sendconnector –identity ‘Windows SBS Internet Send SERVER01’ –port 2525

If necessary, you can find the identity (name) of your Send Connector(s) from the Exchange Management GUI, or from the Exchange Management Shell.

In the GUI, expand Organization Information, select Hub Transport, then click on the Send Connectors tab.

In the Exchange Management Shell, run the   get-sendconnector   cmdlet to get a list of send connectors.