Firstly, unfortunately you need to move away from this product, it is no longer being developed and hit end of life in December 2006. The software is old now and doesn't contain the technology you need to resolve your problem. I suggest you move to the Symantec Brightmail Gateway which you are entitled to do. Once you get upgraded, there are a few things you can do:
1. If you are sure no messages from your domain will ever hit the inbound mail IP address you can add your local domain to your Sender Groups, Blocked Senders domain based list.
2. You can look to create an SPF or Sender ID record and then enable Sender Authentication on your own domain in the Symantec Brightmail Gateway.
Hope that helps,
Kevin