Firewall advice needed.

I need to configure a firewall machine to link some laptops to a network, I am not allowed to alter any settings on the laptops, but will need to divert all outgoing port 25 connections to our mail server. transparently proxy http & https trafic to be routed through an external proxy server. (this is the only way past a external firewall.) Can anyone suggest a good starting point preferably using suse 9.1? Can I acheve redirection of the https traffic? port 25 should just be a iprule command but the proxying is stumping me at the moment. Any suggestions? Rob Keeling