Fedora is a great distro and I use it every day. My main computer is a dual boot FC3/FC4 box. But for a mail server, I'd be very inclined to use Debian. It's not that Fedora can't be a mail server, but with the very quick release times, about every 6 to 9 months and loss of support just a few months after that, it has a rather short lifetime and you'd be upgrading pretty often.
Debian is rock solid and has a much longer release life.
Just my $.02
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