You probably need to take a walk on the dark side of spamming and see 
how the spammers spoof their sending IP address and forge their headers. 
There used to be a department at MIT CSAIL working on detecting and 
squashing such activity. I don't know what ever happened with it.

George wrote:
> Oh here is what I want to do, I have multiple IP addresses on my server, I
> want to setup 4 domains in surgemail so each one is on a separate IP
> address.
>
> I got it all working the way it's supposed to except all outgoing mail goes
> from the same IP address instead of each from it's own IP address. This is a
> problem because if someone decides to do something stupid and gets
> blacklisted it affects all the domains.
>
> How do I make each domain use it's own IP and FQDN to send outgoing mail?
> What did I miss?
>
> George Roettger
>
>
>
>