Which email server you wish to use? there are many choices. http://www.hypexr.org/linux_mail_server.php
plus sendmail which is not the simpliest. If needed so others from the internet can email you to your personal email server then DNS is needed to be installed as well.
Search for email servers for linux on google to see what is out there and what fits what you need.
Also is Redhat 9 new install and new to you. If so you might look into installing something newer in the Redhat clone series like Fedora Core 5 or Fedora Core 6. FC5 would be better. The benefits will be more up to date programs. Going from the older 2.4 kernels to the latest 2.6 kernels. If you run into issues you will have less to have to update.