Honestly, I would rather create a port redirect, rather than change the port which the mail server listens on. Then, you simply change the port within your mail client. I have some IPTABLES examples on how to create Port redirects:
Add the Iptables rules to create the redirect so that any traffic going to port 26 is redirected to 25. In your mail client, configure SMTP to connect to your server on port 26.
Let me know if you need any help.