Redirecting ports

Currently I have a need to forward port 26 to port 25 for tcp only. The machine does not function as a NAT box. So, the machine has 1 ip address to the internet. I still need to be able to connect to 25 as well. I'm having no luck doing so. I've playing around with the rules without completely success. Has anyone come across a rule that might help me out? TIA

David Benigni