Setting up a mail server on port 587
When you set up your mail server for sending you might find out that your internet service provider has blocked port 25. In this case it is useful to use port 587 instead. Port 587's application is called submission. To do this, add the following line to /etc/postfix/master.cf:
submission inet n - n - - smtpd
and restart postfix:
Then you set up your mail client to do TLS (accept all certificates) via port 587 and require authentication.