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Author Topic: Changing SMTP ports in Linux  (Read 3391 times)

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Changing SMTP ports in Linux
« on: June 09, 2018, 06:46:57 pm »
Due to a very large volume of junk email being produced today, most ISPs now block port 25. By default, your server may use port 25 to send mail; as a result, you could find that your email is being blocked. After completing the operations below, don't forget to restart your mail server!


If you have WHM/cPanel, you can simply go into Service Manager and change the port there. Be sure to enable 'Exim on Another Port'.

Otherwise, modify the following file, using your favourite text editor:

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You will need to add these lines, replacing 26 by 587 if desired:

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daemon_smtp_ports = 26
acl_smtp_rcpt = check_recipient
acl_smtp_data = check_message


You will need to edit the following file:

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Add a line to the file in the following format:

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<port> inet n - - - - smtpd
Replace <port> with the appropriate port number, for example:

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26 inet n - - - - smtpd

You will need to modify the following file:

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Add the following line, replacing the <port> with the actual port:

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smtp_alt <port>/tcp # new SMTP port
If you are running xinetd as a super-server, you will need to follow these extra instructions:

First, make a backup copy of smtp_psa by using this command:

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cp /etc/xinetd.d/smtp_psa /root/smtp_psa.backup
Everything located in /etc/xinet.d/ is loaded as xinet services.

Next, edit the file /etc/xinetd.d/smtp_psa and add the following line:

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service smtp_alt
Then restart xinetd using the following command:

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/tc/init.d/xinetd restart
If that does not work, you can use iptables to make the modification:

Change the IP in the following file:

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Then execute this command:

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/sbin/iptables -t nat -A POSTROUTING -o eth0 -p tcp --dport 25 -j SNAT --to $(< /var/qmail/control/outgoingip) >> /etc/rc.local
Verify you are still able to send emails. If not, just remove this line from /etc/rc.local/