Well speaking from experience (a FC1 system), I can say that qmailrocks
has been an invaluable resource to me. It has step by step instructions on how to set up a qmail server with virus protection and spam filters. As far as I know it only has instructions up to Fedora Core 2, but it shouldn't be all that different for FC3.
The major roadblock here is going to be that your ISP may have port 25 blocked (but there are ways around this).