SuSEfirewall 2 port redirection / forced mail server

hi, i want to acomplish the following and wanted to know if its possible with susefirewall2/iptables: our company network is masqueraded by a susefirewall (all private ips). i want all users behind the firewall to use my external mailserver (is virus and spam protected), no matter what mailserver they have in their outlook express settings (i.e. port 25 has to be redirected). the redirect rules of susefirewall2 allow only for redirection to a local port on the firewall, but i don't want any services on the firewall. so i want to redirect to an external ip address (public ip). can this be done (or has it already been done by someone) ? any help appreciated. alex