# Outsiders will never find the MX recors for your domain, because there are no .org servers pointing to your DNS server (you haven't paid for it).

# Even if they did - your local network is using IP addresses on a private subnet, which is not routeable on the internet, so it cannot be reached from the outside.

Postfix should be installed by default. If it isn't - go ahead and install it. Install also the netstat application (use yum search to find the package name) and the telnet command.

Which one is it? Find the port used by SMTP, and look for connctions with the state LISTEN (i.e. currently listening).

Connect from your server to your server using telnet: