If you're looking for just about any sort of security-related (VPN, IDS and other) packages for RedHat 9, Spenneberg has most of the more useful (and recent) ones available in RPM and SRPM format for RedHat 9.
The key aspect (for me at least) is that they apparently aim to provide first-class support of Outlook clients right alongside the others, which lends a lot of credibility to their effort. Of course, using Outlook (not Outlook Express, which is an entirely different beast) against IMAP servers is fiddly to say the least, and there are all those neat message tracking options to deal with...
But this time, I think they're on to something. I've seen a lot of groupware stuff (I used to evaluate products for Eurescom), and most Exchange "replacements" failed primarily due to the fact that nobody had a client as slick as Outlook (Lotus Notes was vastly superior to Exchange, sure, but look at the crappy interface they keep flogging on users...)