Adding additional SMTP listenerports
By default postfix run on port 25 and 587(TLS). However some ISPs block port 25. In that case you can configure the postfix mail server to listen on addional ports too, for example port 26 or some random 5125.
This configuration is done in the master.cf configuration file. Edit it in your editor of choice.
This file is in the following format:
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (yes) (never) (100)
The first column is the port number that you want to listen on. The default SMTP port 25 line will read as follows:
smtp inet n - - - - smtpd
To add an additional listener port of 5125, insert the the following after the above:
5125 inet n - n - - smtpd
Save the file and restart postfix service
service postfix restart
Now you can use port 25, 587 and 5125 to connect to your mail server.