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 additional 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)
# ========================================================================== Continue reading…