SBS 2003 Pro and Sonicwall e-mail appliance

I have a existing SBS2003 network. I want to add the Sonicwall email appliance. I am having trouble with the routing of the port 25 traffic to the appliance. I have a single public IP. The server has two nic cards.

A step by step for routing port 25 traffic to the sonicwall and then to exchange. Now all 25 traffice is routing to exchange directly. How do I change that route? Thank You in advance.
Doug