Exchange Server Alternatives: Update

I have been conducting some research into which platform will be the most appropriate to put in a system that will provide much the same functionality as Microsoft Exchange, without the cost and security vulnerabilities (which may or not be mythical; there is no question as to the cost - it's very real). This is the first of my DIY projects that I approach with some reluctance, as I personally do not use or like Outlook. However, many of my clients, contractors, etc. do, and I can understand their desire to have this functionality in place. So far, it looks like there are two viable alternatives that will do what I want, without serious limitations: Zimbra, and Open-Xchange. Both work within a browser, and permit use from other applications (most notably Microsoft Outlook).

I have a lot more work to do before I make my final determination, including setting up a test environment, some additional research, and serious testing, but I believe I have narrowed it down to these two.