Open ports in the Linux firewall to access POP and IMAP mail servers

To access services such as POP and IMAP mail servers, you must open certain ports to allow the services through the firewall. A few standard ports are used to access most services. For example, access for a website generally uses port 80 for normal (HTTP) web pages and port 443 for secure (HTTPS) pages.

The following table lists the ports used be each of the TCP/IP protocols for mail delivery.

* Although SMTP generally uses port 25 for connections, port 587 is the preferred port for outbound SMTP traffic because of the widespread abuse of port 25.

Edit firewall rules

For Centos 7 and later:

1. Enter the following commands to open the preceding ports:

   firewall-cmd --zone=public --add-port=25/tcp --permanent

   Repeat this command, replacing the port number, for each of the preceding ports.**

2. List the rules on a given zone by running the following command:

   firewall-cmd --query-service=

For Debian and CentOS 6 and earlier:

Edit the iptables.test.rules file to allow access to those ports. Although the example uses port 25, you can change it to the recommended value.

1. Open the rules file with the following command: sudo nano /etc/iptables.test.rules

2. Just before the HTTP and HTTPS entries, add the following lines:

    # Allows SMTP access
    -A INPUT -p tcp --dport 25 -j ACCEPT
   
    # Allows pop and pops connections
    -A INPUT -p tcp --dport 110 -j ACCEPT
    -A INPUT -p tcp --dport 995 -j ACCEPT
   
    # Allows imap and imaps connections
    -A INPUT -p tcp --dport 143 -j ACCEPT
    -A INPUT -p tcp --dport 993 -j ACCEPT
   

3. Apply the new rules:

   For Debian:

     sudo iptables-restore < /etc/iptables.test.rules
   

   For CentOS 6 and earlier:

   Skip this step and proceed to the following step.

4. Check that the rules have been applied:

     sudo iptables -L
   

   The following information should be in the output from the command:

     ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
     ACCEPT tcp -- anywhere anywhere tcp dpt:smtp
     ACCEPT tcp -- anywhere anywhere tcp dpt:pop3
     ACCEPT tcp -- anywhere anywhere tcp dpt:pop3s
     ACCEPT tcp -- anywhere anywhere tcp dpt:imap2
     ACCEPT tcp -- anywhere anywhere tcp dpt:imaps
   

5. Enter the root shell. You must have full root access to save the iptables rules.

    sudo -i
   

6. Save the iptables rules:
   For Debian:

     iptables-save > /etc/iptables.up.rules
   

   For CentOS 6 and earlier:

     iptables-save > sudo iptables-restore <
   

7. Exit the root shell:

    exit