routing mail out on a specific IP address
 

I am hoping someone can offer some help on this. I have a webserver (running Fedora Core 1) and it has several IP addresses coming to it. We run an application on this server and part of the app is it sends out emails to our customer databases. The problem has been that the email seems to go out on any IP address it chooses. Now, some of the email is not getting to customers because the message does not resolve back correctly to the right IP. I would like to set up routing so that email is always sent out on a specific IP address. That way I can be assured that my reverse DNS is always correct on emails sent out.
I am assuming I should use iproute and iptables, I am just not sure how to set up the configuration to make all mail go out on a specific IP address.

Thanks for your help!