As far as which packages to install, I would go with the standard packages (the fewer the better). RedHat has a pre-selected list allowing you to by-pass the task of specifying which specific package to install. It might even be called "Mail Server". The good news is, if you missed one or more packages, you can always install what you missed later using rpm.
But if you insist on getting into the nasty details, I probably wouldn't install any of the "X" packages. The theory being, if this is a stand alone box, which is only handling mail, then there is no need to have a GUI installed.
When you configure sendmail (I'm assuming this is what you will use), pay special attention. There are known security issues with the default properties here, which could allow hackers to use your box to relay their spam.
My last recommendation would be to turn off ftp and telnet, and only use ssh to work on this server. No need to have clear text passwords running around. 😉
Goodluck.
