I want to FORCE users/servers on the network to redirect
outgoing data sent
on port 25 to another server (Internal mail server)
That's a bit overbearing of you. If you really must, block port 25 at the firewall apart from to your relay mail server. This forces the clients to acknowledge that there is One True Email Relay for them.
You can do "clever" stuff with catching port 25 and using an iptables redirect, as freeserve appear to, but why confuse any clients? Just chop their bits off if they attempt to do something that they should not.
Some servers are a reseller style system, so your not in control of all the users. This is the ONLY way to do it and it will stop the network being used for spam
I do agree in most instances but this time its the only way forward.
I figure that ipchains/tables are the way forward
Just wanted to get some feedback before hunting on google for some working examples
Thanks D