This being said, as long as repositories remain in existence providing errata updates I suspect FC1 will be a good solution for your mail server. The advantage is that it is easy to configure and anyway, even if you change platforms in future you can always migrate it to another box running whatever flavour of Linux you want, as long as you use sendmail, in my opinion.
Ditto postfix. My company has approximately 40 users and 130 computers.
Our DNS, LDAP, DHCP and SAMBA PDC and BDC are running (so far happily) on FC1. (The only problem that's come up is keeping the 'ldap' user from running out of open files.)
I have a new FC1 mailserver ready to migrate to using postfix, cyrus-imapd, and spamassassin. (Waiting for downtime to migrate the user data).
I plan on budgeting in the future to put the core systems (DNS, mail, web, etc) on RHAS or RHEL just for peace of mind, but FC1 is running pretty happily at the moment.
Our desktops are being upgraded to FC1, and I'm extremely happy with it in that role.
The compute farm is a work in progress. It has been running at 7.3 for quite some time, but the acquisition of newer hardware, plus high threading and memory requirements from some newer tools, is pushing us to a 2.6.1 based system (currently I install FC1, then update it to the development repository, just to make sure all the subtle kernel-related things get changed together).
I'll probably freeze a particular set of packages as my own 'stable' installation for core/desktop/compute use, then pull in updates on a case-by-case basis. The desktops will probably track actual updates most closely, since that's where people see the most day-to-day improvements, but the servers will end up being much more static.
