Some of our users have ISPs who block access to port 25, so were unable to send mail through our server and were forced to route outgoing mail through their ISP's mail servers instead. Reconfigure routers wasn't an option for us, as the server hosting company didn't allow any alterations to the routers.
After a bit of searching we came across a utility called "PortTunnel" (from http://www.steelbytes.com). It simply runs as a service and re-directs any incoming traffic on a given port to an alternative port. In our case we map port 587 to 25 to get around the ISP restrictions, and just tell the users to change their "Outgoing SMPT Server Port" in Outlook.
The "14 day free trial" version's only main restriction is a limit of 50 simultaneous connections (more than enough for our needs!). Also, the 14 day free trial version is still running strong after many months...
Hopefully this will be redundant once the "listen on two ports" functionality has been added to AMS, but in the mean time this does the job!