I think the Courier-IMAP package has a webmail interface. If you go with it you might want to go with Qmail to replace Sendmail altogether. Both QMail and Courier include a pop3 server as well.
http://www.sxpress.com/~henry/qmail-...-web-howto.htm
(Don't have to use vi to create the files btw, can just use your default editor with a telnet client such as
Teraterm that supports copy/paste). Qmail and programs that were written to work well with it are pretty old and well rounded, as well as well-documented. If you follow the installation instructions *carefully* you shouldn't have many problems at all setting this stuff up.
The above link is a complete guide for setting up Qmail as a replacement for Sendmail, IMAP access, a POP3 server, and webmail using Red Hat.
It's not gonna be a point and click install, no, but it will do everything that a Win2000/Exchange server will do without the inherent Windows/Exchange security problems, and with a pricetag of zero instead of several thousand.
If you wanna administer a server, might as well take the time to learn how, you can't buy everything

.
Yeah it'll cost you a couple of days of reading and testing/debugging, but it's not rocket science and at least you'll know how it works when it needs fixing later down the line.