dymanic routing problem/least-cost routing

[Blotto99]

I have an issue where we have to e-mail/webservers behind our firewall. One was setup a year ago, the other I am setting up now. Both only have 1 NIC, but have an external (valid) IP Address and an internal IP Address. Our firewall does the address translation.<br>
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While setting up this new e-mail/webserver I find that our Exchange Server cannot send mail to it. The e-mail from the Exchange Server MUST route through the external (valid) IP Address of the new e-mail/webserver. It seems that no matter what I put in the TCP/IP Routing Table, the gateway for the e-mail/webserver entry gets dynamically changed from the firewall address (the one I want) to the internal IP Address of the e-mail/webserver.<br>
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The reason I figure, it because the path to the internal IP Address is a shorter route.<br>
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I know that this can work, because the other e-mail/webserver that was set up 1 year ago works great and the route in the TCP/IP routing table never changes. It is static, with the gateway being the firewall. It too has a shorter route via it's internal IP Address, but it doesn't change.<br>
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All help will be greatly appreciated.<br>