Forward SMPT port 25 with other port

Linux does not allow root users to bind port traffic to ports 1024 and below. However, the simple mail transfer protocol (SMTP) has a default value of port 25. If you are running Linux, you must configure the reserve SMTP agent to listen to a custom port instead of the default port 25.

Before you begin
Run the commands in the following procedure as a root user.

Update your firewall to open the port above port 1024 by adding the following lines to your

vim /etc/sysconfig/iptables file:

-A OUTPUT -p tcp -s serverIP -d 0/0 --dport Port_Above_1024   -m state --state ESTABLISHED -j ACCEPT
-A INPUT -s 0/0 -d serverIP -m state --state NEW,ESTABLISHED  -p tcp --dport Port_Above_1024 -i eth0 -j ACCEPT

Restart your firewall:

/etc/init.d/iptables restart

Open port 25 for forwarding:

iptables -A FORWARD -p tcp --destination-port 25 -j ACCEPT;

Forward port 25 to your custom port above 1024:

iptables -t nat -A PREROUTING -j REDIRECT   -p tcp --destination-port 25 --to-port Port_Above_1024

To verify that port 25 is forwarding, Run a telnet command:

telnet yourServer 25

If you do not receive a successful response, forward port 25 to a different custom port.

Port_Above_1024 change the same to any port no. above 1024