ISP blocking port 25? No problem!

Is your ISP (cough* rogers * cough) blocking port 25 and preventing your users from using their local email client (Outlook, Outlook Express)? Well, no need to worry. I think the simplest way to get around this problem without having to reconfigure any mail server settings is to allow your users to enable port 26 instead.

Lets start by opening up port 26 on the email server:
iptables -A INPUT -p tcp --dport 26 -j ACCEPT

Secondly, we will port redirect all traffic from port 26 to port 25:
iptables -A PREROUTING -t nat -p tcp --dport 26 -j REDIRECT --to-port 25

Damn, that was easy!