Your hardware specs exceed that of the box that was serving mail to
*more* students about 8 years ago at Duke University. That box, if
memory serves, was a Pentium 90MHz running Solaris x86. Though it did
have the benefit of SCSI disks.
If you can serve only POP3 instead of IMAP, that box will have no
problem serving that many users and more.
If you're going to do IMAP, I suggest trying out Dovecot and doing some
comparative benchmarks before you settle on Cyrus. I made the switch
on a dual Pentium III 800MHz box, 256MB RAM, 20GB disk, and the
performance difference was staggering.
I agree with using Postfix instead of Sendmail.
Some of the other things like virus scanning and spam filtering is
something the old Duke mail server never had to worry about, and
admittedly that adds a big unknown into the equation.
I'll echo the sentiments of the other folks and say that /usr needs to
be bigger. Also, I highly recommend putting everything but /boot into
LVM so that you can take advantage of more disk later on.
