When I did something similar to this a couple of years ago. It was different in that I was rerouting incoming mail, not outgoing. However, you should be able to accomplish what you are wanting using similar settings. I'm going to shoot from the hip on this (I don't have a copy of Exchange to look at), so please bear with me if I'm a little off on tabs and such.
The simpliest way to accomplish this would be to reroute the messages using the IMC in Exchange. In the IMS properties page, on the "Routing" tab, you can reroute email. Click the "Add" button and add abc.com to the list and tell it the IP address that the mail for abc.com should be rerouted to.
I also seem to remember something about the Connections tab. There is a button in the Message Delivery section that allows you to specify where to deliver messages destined for a specific domain. Click the button and see if it applies to your situation. <>
Depending on your setup, you may have to play around with the Address Space tab also. Most installations have a wildcard to allow it to accept messages to any domain, so it may not be an issue for you.
Anyway, play around with the IMS. It can accomplish what you are asking. Good luck!
