How do I re-route an IP address?

How do I forward or redirect a requested IP address to another IP address. Or is that even possible?

The reason I’m asking is because we have a SonicWall SOHO3 firewall appliance with NAT enabled. We’ve configured it so that the outside world can get to our mail server (which is behind the firewall on the LAN) using a public IP address. This works great, btw. The problem is that if I ping this public IP address from within the LAN itself, it goes nowhere. I want to be able to redirect local request to this public IP address to our mail server. In other words, I want to be able to access this mail server from within the LAN using either local or public IP address. How can I go about doing that? Thanks in advance.