This is a pretty ambigous (sp?) question, however, I will attempt to answer it.
The majority of distributions come with a mail server already setup and ready to go. When you refer to "secure it" what exactly do you mean? If you are running a local network and wish to restrict access from anyone on the internet to access the mailserver to retrieve, then you can just filter out the ports nessesary (such as 110 for pop). If you would like to continue to keep access open to the outside network, then you can use somwthing like uw-ipopd which has built in SSL (secure socket layers) support for encrypted information being retrieved. Redhat, I believe, comes with sendmail already configured and ready to go, so that can be used to send your email.
For a more detailed description on how to get this running, please refer to
the mail administrators HOWTO.