It may be when you try to send test emails, they doesn’t arrive. So the first thing we do is to see the logs. File /var/log/mail.log should give us a clue and when trying to send a message we can see clearly this “permission denied message”.
It means postfix doesn’t belong to mail group, so, let’s make it:
$ sudo gpasswd -a postfix mail
Then restart postfix, and you should start receiving messages.