Looking for a Linux-friendly groupware suite that can take the place of Microsoft Exchange in your organization? You'll find a wide range of alternatives for Linux that offer most (if not all) of Exchange's functionality.
If your organization has standardized on Microsoft Exchange, switching may be a bit tricky (but can be done). But if your organization hasn't started down that path, it's a good habit to avoid. The good news is you'll find several robust Exchange alternatives for Linux.
The Core of the Matter
One of the things you'll run into quickly with enterprise open source offerings is open core offerings — meaning some of the software is open source, other bits are under a proprietary license of some kind. Usually connectors to Microsoft Outlook and mobile synchronization are held back, or features that are primarily focused on the enterprise — like clustering to provide scalability and failover.
Typically these costs will be much less than actually deploying Exchange, so organizations that need them are happy to pay the licensing. The downside is that they're not actually open source, so you lose some of the benefits of open source when choosing an open core solution. I'm not going to hop on a soapbox either way, but it's worth pointing out that when you're talking enterprise open source be sure to verify which features are open and which may be held back for licensing fees.
Citadel is one of the pure open source offerings. It provides email, calendaring and scheduling, a Web interface, mailing lists, address books, LDAP authentication, and quite a lot more. It's focused on standard protocols (SMTP, IMAP, POP3) and offers just about all the features that a small or medium-sized business would want. There's no "enterprise" version, everything offered with Citadel is under the GNU General Public License version 3 (GPLv3).
You'll find several ways to install and test Citadel. The downloads page offers an "Easy Install" package, binaries for Debian and Ubuntu, a pre-built VMware appliance, an eBuild for Gentoo, and (of course) source code.
If you're looking for groupware, Citadel is a good choice. If you want full Microsoft Outlook compatibility, you'll need to also use the Bynari Outlook Connector, which is not open source. Of course, if you're also using Outlook, you're not working in a 100% pure free software environment anyway. Clients like Evolution and Kontact can work with Citadel using GroupDAV.
My favorite bit about Citadel, aside from the open sourceness? It features an old-school text Bulletin Board System (BBS) interface. You won't find that in the others.
The Web site for Citadel is a bit outdated, and it's not clear how active the project is. The last release was in May of 2009, but the mailing list seems active and packages for Citadel ship in Ubuntu 10.04 so it should be a reasonable option even if it development isn't moving too quickly.
Kolab is another groupware contender that's fully open source. It integrates Postfix, OpenLDAP, Cyrus IMAP, Apache, HORDE Webmail, ClamAV, and a number of other open source offerings into a single offering. You get calendaring, email and Webmail, Web administration, shared contacts, and more.
The most recent release came out in July, with the 2.2.4 Kolab server release. Kolab packages are also available in Debian and Ubuntu, and up to date packages are available for download as well. The project has plenty of documentation available if you happen to speak German. The Administration manual is only available in German at the moment, though the install guide has an English version. The wiki also has a fair amount of docs as well.
Thunderbird and Kontact are Kolab-friendly clients, and several proprietary connectors are available for Microsoft Outlook if you have Windows users to look after.
Kolab might take a bit of elbow grease to configure, but should be a solid groupware suite for small businesses.
Open-Xchange has been around for quite a few years and provides an alternative to Microsoft Exchange as well as some Microsoft SharePoint features too. It offers email/Web mail, calendaring, task management, document management, and more.
You'll find several editions of Open-Xchange, depending on what you're looking for. Small businesses will probably want the community edition or Appliance Edition. The Appliance Edition is an install that provides everything from the OS to the Open-Xchange software. The company also provides hosting editions for hosting providers.
One of Open-Xchange's interesting features is its plugin architecture. You can "OXtend" Open-Xchange (their term, not mine!) to do things like gather information from social sites like Facebook, authenticate against LDAP or IMAP servers, and more.
Licensing is a bit tricky for Open-Xchange on the community side. Its server-side software is under the GPLv2, but the Web interface is under a Creative Commons license that doesn't allow commercial use. So if you're putting up a an Open-Xchange system for your business or project, that should be fine with the community edition. If you want to offer hosting with Open-Xchange that you charge for, you're going to have to license it from Open-Xchange.
If you don't want to deal with hosting Open-Xchange yourself, the company has a number of partners that offer hosting solutions using Open-Xchange. This might be a good tradeoff, since you can always switch providers or start hosting on your own later if your organization grows out of hosted groupware or if you decide that you're unhappy with the current provider.
Finally, there's Zimbra. Zimbra has a bit of an interesting history as a company. Zimbra was originally produced by a small independent company, then purchased by Yahoo! in 2007. Not content to be acquired once, Zimbra was then spun off by Yahoo! to become a division of VMware. So far, the ownership changes don't seem to have affected the project too much.
Zimbra provides Web-based email with a slick AJAX client (remember when AJAX was a buzzword?), standard email, calendaring, mobile email, document management, and task management. As with Open-Xchange, Zimbra comes in several flavors — an open source edition, hosted editions with additional features, and a pro edition that bundles Outlook sync and email archiving features that might be important for enterprises. If you need mobile sync, clustering, backup and restore features, you're going to need to look at one of the non-open source editions.
You'll also find a Zimbra Desktop client that provides a great interface to Zimbra when you're offline. Like the client, but don't use Zimbra? No problem. It works with other mail providers too, so you can configure the desktop client to work with Gmail, your ISP, your work email, or whatever. It runs on Linux, Windows, and Mac OS X, so you should be able to support just about any user with Zimbra Desktop.
There's plenty of offerings for Linux if you want to avoid deploying Exchange, or if you're looking to migrate away from Microsoft Exchange. They're not all open source, but you should be able to find a solution that fills the feature requirements at a lower cost per head than Exchange, running on Linux.
Whether it's a 10 person shop or a company with 1,000 employees, the Linux Exchange alternatives should fit the bill.