Server :: Setting Up Postfix To Use Multiple IP Addresses?

I set-up his mail server for him with Postfix but what he wants I have no idea how to do.

Essentially he wants it so that the mail gateway IP corresponds with the dedicated IP of the domain and I have no idea how to accomplish this.

I found some documentation on the web saying that the fix for this was to run multiple instances of Postfix which I tried doing but each time I try to start the second instance I get the error that postfix is already running.

There has to be an elegant way to make this happen, I really hate to tell a client something can't be done even though the concept is a bit pointless, IMHO. I am hoping I can get some feedback here on if this can be done and if it can the easiest way for me to accomplish doing it.

Here is some of the config files (example.com has been put in place of the actual domain names and the ip of 5.5.5.5 is in place of the actual IPs


Code:
[root@youronlinehosting ~]# cat /etc/postfix/transport
example.com smtp:5.5.5.5
example.com smtp:5.5.5.6
example.com smtp:5.5.5.7

[Code].....