Just incase anyone is wondering if just running yum install postfix will be enough to get postfix up and running for you, for me it was not...

you will need to get

yum install redhat-switch-mail

then run

/usr/sbin/redhat-switch-mail

and switch it over to postfix (or exim if your using that).

You can follow this howto here: http://www.fedora.us/wiki/SwitchingToPostfix

And there is also another mysql/postfix one here: http://www.fedora.us/wiki/PostfixWithMySQL