Do you want to change the default Postfix port?
Here’s are the easy step-by-step how to change the default Postfix port:
Step 1: Edit the file /etc/postfix/master.cf and find the line below. You can comment the line below by adding # in front of the line to disable SMTP port 25.
smtp inet n - n - - smtpd
Next, add this line
25000 inet n - n - - smtpd
Replace 25000 with your preferred alternate port number.
Step 2: Restart the Postfix service or the MailScanner service if you have integrated MailScanner into Postfix.
Step 3: If you have enabled the firewall, you need to configure the firewall to allow traffic on the new port number.
Step 4: Test Postfix by connecting on the new port number. Use the terminal command telnet localhost [port] instead of telnet localhost smtp to test your new port number.