If you're looking to run a serious open source collaboration server, Yahoo's Zimbra Collaboration Suite (ZCS) should be on your short list. This Web 2.0 email and groupware server offers AJAX Web-based administrator and user interfaces, a variety of useful groupware features, and email import functionality.
ZCS comes in five versions. The Open Source Edition, which is the one I tried, doesn't have all the features of the others, but it's purely open source.
Zimbra claims all the editions run on Mac OS X and a variety of Linux platforms, including Red Hat Enterprise Linux (RHEL), Fedora, Ubuntu, Debian, Mandriva, SUSE Linux Enterprise Server (SLES), rPath (a software appliance ISO image), and VMware (a certified virtual appliance). I installed ZCS 5.0.20 on both openSUSE 11 and SLES 10 SP2.
Installation was a breeze, and that's something I rarely say about serious server applications. Zimbra uses a single staged installation to install all its multiple parts. Like many Unix and Linux applications, Zimbra actually incorporates multiple applications. It uses Apache Tomcat for the Web application server, Postfix for the mail transfer agent (MTA), Clam AntiVirus for virus scanning, SpamAssassin and DSPAM for spam filtering, OpenLDAP for user authentication, and MySQL for user preferences and the message data store. You could install all those by hand, but who would want to?
Your server must have several other common Linux utilities installed as well. The only ones you might not already have set up are cURL, a shell tool for transferring files using URL syntax; the fetchmail remote mail transfer and forwarding utility; and the Libidn library to handle international email addresses.
As for the server hardware, Zimbra recommends a 2GHz or faster 64-bit processor with at least 2GB of RAM. The bare bones for storage is just over 15GB of hard disk space, but that understates real-world requirements. Zimbra doesn't say how much room you should set aside for users' email storage, but I recommend at least 500MB per user. That might seem excessive, but with users now in the habit of attaching large files and letting their mail sit on the server, 500MB is reasonable.
This leads to my first serious concern about Zimbra: the default installation is for one system to handle the entire load. That means a lot of applications are depending on a single system to always perform quickly. I'd prefer to split at least the database and the storage functionality from the application and email server. Zimbra seems to be aware of the concern, because it recommends that you not use RAID 5 if you have more than 100 accounts. Why? Because if you have more than that, your users are likely to see significant slowdowns due to excessive disk activity. It's just too much load for a single system using RAID 5 storage. Clustering, of course, would help with this kind of load issue, but that, along with high-availability support, is only available with the commercial editions.
On my test systems -- a pair of Hewlett-Packard Pavilion a6040n Desktop PCs with 1.86GHz Intel Core 2 Duo E6320 processors, 2GB of RAM, and 320GB SATA drives -- Zimbra ran without any hiccups. However, I was running it with a light load of no more than 20 users at any given time.
The lively and helpful Zimbra Forums are another plus for small businesses and groups. Relying on mailing lists and forums for support is all too often a hit-or-miss proposition, but with Zimbra, it's a hit. The forums are active, and questions are answered promptly by Zimbra staffers and experienced users who know Zimbra well.
Once I had Zimbra up and running, I found managing the system to be a snap. The Web-based administrator interface makes it easy to set up users and system-wide defaults. For example, you can set up Zimbra so that it automatically dumps incoming email with .exe attachments into the binary garbage bin.
Zimbra also does a good job of working with Outlook 2003 and earlier with its Zimbra Connector for Outlook. This does not, however, work with Outlook 2007. A beta Connector addresses this concern, but it doesn't work that well. In any case, the Connector for Outlook and the Connector for Apple iSync are only available with the proprietary versions of the program.
Of course, what Zimbra really wants you to do is use its Web-based Zimbra Desktop client. This AJAX-based email and calendar application is available for Windows, Mac, and Linux. It's attractive and easy to use, but I still prefer standalone email clients such as Evolution. You can hook up with Evolution using the free Zimbra Collaboration Suite Evolution Connector, but since both Zimbra and Evolution natively support POP, SMTP, and IMAP for email, and Webcal for calendaring, I'm not sure what the point is for this connector.
The Open Source Edition also lacks a built-in backup and restore facility. That's a major negative for anyone who wants to use Zimbra for serious work. The other editions do include this functionality.
Zimbra's Open Source Edition is released under the Yahoo! Public License. This is a variant on the Mozilla Public License and is not recognized by the Open Source Initiative as an open source license. This difference won't mean much to most users, but it's worth noting for those who prefer their open source licenses to be officially open source.
All in all, the Open Source Edition is a good way to try out Zimbra. Without backup or clustering support, I can't recommend this edition for business use, but the commercial versions look like they would work well. It is annoying, however, that despite all its virtues, the Open Source Edition is almost crippleware.