The default SMTP port is 25. In Postfix we can change it to some other port (say 2525) using the following steps.
Open the file master.cf.
$ vi /etc/postfix/master.cf
Add the following line to the file.
2525 inet n – n – – smtpd
$ /etc/init.d/postfix restart
You can check the connection to the new port using telnet. Also make sure that the new port is not blocked in the server firewall.