When I set up the mail server, I had some interesting problems.
Fedora Core 2 comes with Sendmail installed by default. I like using Postfix, so installed a Postfix and redhat-switch-mail. I ran redhat-switch-mail, and switch the MTA to Postfix. I then configured Postfix to only not relay mail spam mail, and only accept mail for our domains.
When I started Postfix, I noticed that a bunch of spam messages were getting through and being relayed. I immediately shutdown Postfix so I could figure out what was going on. It looks like the redhat-switch-mail program didn't actually removed the Sendmail xinit.d file. So the Sendmail was accepting messages without any filtering, and was forwarding it to Postfix. I immediately disabled this script.
In addition, I wanted to enable TLS for use for outgoing mail. Unfortunately, Postfix has to be rebuilt with a patch. I didn't want to rebuild it, as I wouldn't be able to upgrade using rpm. It is unfortunate that there isn't an rpm that has TLS enabled by default.