Hi dears

I have one solution but i confused so bad for installing the options.

u can see the plan in the pic.
mpf2006.persiangig.com/2m/Drawing1.jpg
i have a local mail domain.
i want to change the local mail domain(that have private ip) sender into main mail domain(that have public ip) sender and send it over the internet.and also i want to forward mails from Main Mail Server into Local Mail Server

any body has any idea?