The default SMTP server used in the current release of RHEL is Postfix; the original Sendmail package is still available, but is no longer the default.

It's normal for the e-mail server to listen on the localhost or loopback address only when using default settings. In this way, local e-mail delivery is possible out of the box. This is used by many services, including the crond job schedule manager. If we wanted to enable Postfix to listen on all interfaces and receive messages from remote systems, there is a little configuration required.

Firstly, we will back up and tidy the main configuration file. There is a tendency for many software packages to over comment their configurations. This can cause issues where you think that you have implemented a change; however, it was also set later on and you may not have noticed it. There are 679 lines in the default configuration file: /etc/postfix/main.cf. We will back up the file so that we do not lose comments...