It's non-trivial. There's a good postfix guide on the CentOS wiki, though I'm not sure how much of it applies to Fedora. You set up a server, then you have to arrange for that server to have some sort of official address, such as using dyndns or no-ip.com, then set that up with a domain registrar like namecheap.com.
Otherwise, mail coming to you from outside won't know where to go.
http://www.linuxhomenetworking.com/w...r_Own_Web_Site
gives an introduction to hosting your own site--even though they're talking about a web site, it's the same issue, it has to be available to the outside word.
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