I don't think there is a gui to configure a mail server for Redhat .. yet .. so it doesn't matter if you are using KDE or gnome.
From smtp you need to install "sendmail" package and for pop3 services you need to install the "imap-2001a-15" package.
Then you probably need to edit these files -
/etc/mail/local-host-names
enter you domain name here to be on the save side. eg.
abc.com
/etc/mail/access
should have something like this
localhost RELAY
127.0.0.1 RELAY
192.168.1 RELAY
Last entry for a whole class C that matches 192.168.1. Note should be replace by you local address.
That should allow you to set up a basic system ...
Anything more advance you can try the sendmail document at www.sendmail.org