Have you tried sending e-mail from another domain to your domainname address.
You should get one of your friends to send you e-mail from their external site to your domainname address and see if that works, and if it does not, to examine carefully the bounced back error message.
On your server, are you blocking port 25 with any firewall rules?
Have you looked in /var/log files, using grep, to see if there have been any connection attempts to port 25 from external mailers?
Do you have sendmail running as a daemon or as an inetd or xinetd started service, and is it configured in sendmail.mc to listen on the IP interface rather than just the loopback interface?
In fact have you been able to receive any external e-mail at all?
As regards to a guide on Red Hat 8, you should look in
/usr/share/doc/rh-* directories which cover aspects of system and network configuration, including sendmail.
For more detailed information, look at the README.* files in /usr/share/doc/sendmail-{version_number}.
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