It depends on what email server software you have installed. Since you're using Red Hat, I'd have to assume you're using sendmail.
If memory serves, you configure sendmail by editing a certain file (sendmail.mc), then running a conversion script (don't remember the name), and that script translates your changes from the sendmail.mc file to sendmail.cf (which is what sendmail actually reads when it runs). Why is it done that way? I dunno... that's partly why I use Qmail
Anyway, check to make sure you have the sendmail-cf package installed. For whatever reason, there are two packages: one for sendmail itself and one to allow you to configure it. The sendmail package is included by default if you select the "Mail server" option through the GUI package manager. The sendmail-cf package is located slightly below, and is not included by default. So you'll have to manually select it.
After installing (if necessary), search for the sendmail.cf file (can probably just do a locate sendmail.cf
). The sendmail.mc file should be in the same location. You'll have to find a sendmail tutorial to find out what the conversion program is, or maybe someone else here will chime in with specifics. You'll probably need the tutorial to explain the options available as well. So I'd hunt one down regardless.
On the other hand, if you're using qmail, you configure it by modifying the files in /var/qmail/control (default location).