We're talking small-potatoes here :) Maximum 50 users, light load: about 30 messages delivered to local inboxes each hour. I've been running on older hardware for a long time and will continue to do so for a while as I don't think I'm maxing out what I have so far: Dual PIII 1GHz, SCSI disks mirrored w/ RaidFrame RAID1. Given your needs, I think the bigger concern is reliability. I would use RAID-1 and turn on wapb on the data filesystems, leaving it off on /, and I'm a believe in a separate / /var /usr /home setup especially for a mail server. Also, I'd use dovecot, but cyrus-imapd might be ok.
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