On servers I tend to stick to distro packaged software where possible, since it reduces the maintenance overhead a lot.
If you are using Fedora then the postfix MTA is included as an alternative to sendmail. Postfix is nice, and will scale to your requirements, but if you envisage a complex mail-routing arrangement then exim (in Extras) would be more appropriate.
For really big IMAP systems Carnegie Mellon University's Cyrus is the most popular choice, and is now packaged as well.
I've setup the Horde with IMP, and was very impressed with it, although it was a pain to put together. Fedora ships with Squirrelmail for Webmail, but for a large deployment IMP would be my choice as well.
If you haven't already done so, it's well worth looking at Novell's Hula, which provides SMTP, IMAP, Webmail and calendaring, and is insanely scalable. Unfortunately the first stable release is scheduled for September, but once it's ready I suspect that it will rapidly become the default answer.