I have a dedicated server, and the way it was set up by the provider, they gave eth0 an IP address, which they say could change from time to time. I have 8 IPs that I can use, and they're mapped as lo:1, lo:2, lo:3, etc. I'm only going to use 1 IP address (lo:2), so I've disabled the others.
I'm going to run a mail server (which I've already set up, QMail for those interested), but when I send messages, in the header of the email it says it's from unknown HELO at eth0's IP address. I would like to use the IP of lo:2, but am not sure if this can be done. I know the receiving mail server is detecting the IP it's being sent from, but is there a way to make it send from the IP at lo:2? I tried using iptables to forward the address:25, but it didn't change. I may have not written the rule right, but I'm starting to think I'm just SOL.
Any suggestions?

I'm not sure but doesn't Qmail allow you to configure what IP address to connect to?

might want to check your reverse DNS on your default domain and the me file in the /var/qmail/control directory, although qmail can listen on several ports and IP addresses it usually sends out from the default IP.