By default, Postfix listens for connections on port 25/tcp and in this article I will show what needs to be done so that it starts to happen port 587 (submission), since for example in many mail clients when setting up mail, port 587 is offered by default.
Open the configuration file /etc/postfix/master.cf in any text editor:
sudo nano /etc/postfix/master.cf
Find the line below and uncomment it:
submission inet n - n - - smtpd
Under this line, you can also specify some parameters (there must be a space before each line):
-o syslog_name=postfix/submission -o smtpd_tls_security_level=encrypt -o smtpd_sasl_auth_enable=yes -o smtpd_client_restrictions=permit_sasl_authenticated,reject
If a firewall is used, then do not forget to open incoming connections on port 587/tcp.
Check the configuration and restart Postfix to apply the changes:
sudo postfix check sudo systemctl restart postfix netstat -na | grep LISTEN | grep 25 netstat -na | grep LISTEN | grep 587
See also my article:
Installing and Configuring Postfix