I had a problem where my postfix mail system wasn’t listening on its IP address 10.1.1.123 but it was listening on 127.0.0.1. I checked my firewall settings and made sure port 25 was open, but I still couldn’t connect.

I read an article, Bind Postfix Mail Server To Localhost or Specific IP Address Only, which gave me the hint I needed.

The trick was to comment out inet_interfaces in /etc/postfix/main.cf because it was specifying loopback-only which meant postfix wasn’t listening on its other IP addresses.