Exchange Server Alternative: Results so far

So I've been working away, trying to identify the best alternative to Microsoft Exchange Server for my organization. I had initially narrowed the field down to two: Zimbra, and Open-Xchange. I've spent a few hours today going through the latter, and a fairly thorough review of the Open-Xchange site suggests that it supports the following:

• Messaging (email)
• Shared Calendars
• Shared Tasks
• Shared Contacts
• Document sharing
• Project portals (not entirely certain what this is, but if it is what I think it is, it looks promising)
• User forums, bulliten boards
• Knowledge base (sort of like a faq on steroids)
• Bookmark repository (our own private del.icio.us?)
• Support for Outlook (through a nifty api called the oxtender)
• Support for Palm
• Support for Samba
• Support for SyncML
• Advanced user management
• Advanced messaging archival/reporting/retrieval

Wow. That exceeds my initial requirements list, and then some. There are, to be fair, a few missing elements -- spam & virus prevention are must haves, and network faxing would be nice. If my cursory read of the architecture is accurate, then I suspect none of these are going to be problematic (in fact, I might be able to stick my my existing smtp transport layer, and have to do nothing in this regard). This is an extremely well thought out package. I plan on reviewing Zimbra's featureset next, and then doing a test install of each package.