Exchange Server Alternatives

We have been under some pressure from both clients and selected staff (curse them) who really, really want to have an Exchange server installed for our office. Although I have resisted for quite awhile, and continue to use a rock sold qmail installation for our email services, there are, I admit, some benefits to having Microsoft Exchange installed -- shared calendars, shared address books, and so forth. However, Microsoft Exchange is a pricey little thing, and I have heard countless horror stories about security.

Naturally, I am looking for a free, stable, secure alternative right now.

So far, I have come up with this list of possible alternatives:

• eGroupware - a PHP based groupware solution, intended to be used with a web browser. Not a true replacement for exchange server. http://www.egroupware.org
• Group-Office - like eGroupware, this is a PHP based groupware solution, intended to be used with a web browser. Not a true replacement for exchange server. http://www.group-office.com
• Open-Xchange - Hmmm... now this is more like it. Integrated SMTP & messaging server, integration with Samba, and "OXTenders", for connections with various non-browser applications. This sounds more like what I am looking for. http://www.open-xchange.com/EN/developer/
  
• Kolab - From their site: "Kolab is a Groupware Solution for Emails, Appointments, Contacts and more. It supports mixed client environments (Outlook/KDE) because of an open storage format. Any email client speaking standard protocols can be served. For the full Kolab experience you need a Kolab Server and Kolab Clients. " A quick overview of the required software suggests that although this will connect with Exchange, it requires a "proprietary, [connector] with gratis 30 day evaluation", called "Toltec connector 2". Hey, I don't want to spend any money if I don't have to... http://www.kolab.com
• OGo - OpenGroupware http://www.opengroupware.org
• Zimbra http://www.zimbra.com
• Open Source Outlook MAPI Connector http://www.openconnector.org

Update: A visitor suggests Scalix (www.scalix.com), which looks very promising. It comes in two flavours - commercial, and "community edition." The latter is free, but limited to 25 users. I will include it in my research, though.

I have not done a great deal of research into any of these, but plan to do so over the holidays. I'll keep you posted.