How do I use iptables to route to another port?

You can use iptables pre-routing to route from one port to another.

For example, the following command will enable mail users to use port 26 or port 25 for SMTP request. All port 26 requests will be routed to port 25.

# iptables -t nat -A PREROUTING -p tcp -d --dport 26 -j DNAT --to

Now your clients can use port 26 or port 25 without any problems.