How to Allow Port 26 for SMTP in IPtables
Please be aware that some ISPs in Malaysia and other countries are currently blocking SMTP. To reduce spam on their network, they use port 25.
Use port 26 is a strategy that many systems administrators employ so that you can setup your email client and your coworkers. To send emails, email clients should utilise port 26 rather than 25.
Regardless of the server you are using—Postfix, Sendmail, or any other server—here I will demonstrate how to set up your SMTP server to accept email connections on port 26. Insert this line into your firewall setup in order to open port 26 first.
iptables -A INPUT -p tcp –dport 26 -j ACCEPT
You won’t need to change the configuration of your email server to listen on port 26 because this will allow the server to accept connections on port 26, from anywhere in the world, and then redirect the connections on port 26 to port 25.
iptables -A PREROUTING -t nat -p tcp –dport 26 -j REDIRECT –to-port 25
You can configure your email server to accept emails on port 26 by adding these two lines.