Open source tools and e-mail share a long history. Mail servers such as Exim, Postfix, and Sendmail enjoy widespread use, to say nothing of a healthy assortment of open source mail clients, from Mozilla Thunderbird to Pine. But e-mail isn’t the be-all and end-all of enterprise messaging. For advanced features such as group calendaring, shared address books, and IM integration, enterprise customers typically have had just two options: IBM’s Lotus Notes and Microsoft Exchange.

This is beginning to change. Although the Big Two of enterprise messaging are likely to dominate for the foreseeable future, a number of attractive alternatives have begun to appear, particularly for small to midsize enterprises. Not surprisingly, many of these promising newcomers hail from the world of open source.

Perhaps the most ambitious such project, Open-Xchange , replicates the functionality of traditional groupware servers in an open source package. Open-Xchange integrates a variety of open source tools -- including the Apache Web server, the Tomcat Java servlet engine, and the PostgreSQL database, among others -- to offer full e-mail, calendaring, and collaboration capabilities through a Web-based UI. Commercial add-ons allow Open-Xchange to interoperate with Microsoft Outlook and Palm handheld clients.

Still other open source messaging servers take interoperability a step further. Zimbra , for example, uses AJAX (Asynchronous JavaScript and XML) technology to integrate messages with outside data sources, from address book lookups to Google Maps. What’s more, Zimbra was designed from the ground up as an interoperable system of components, many of which are exposed as Web services. Third-party developers can connect to these components’ APIs to gain direct access to Zimbra calendar items, address books, and e-mail.

Because Zimbra is open source, developers don’t have to navigate around proprietary pitfalls. They can get right into the meat of the code and integrate their own software directly. Open source messaging systems also impose no legacy communications interfaces. In place of proprietary Microsoft Exchange protocols, Open-Xchange and Zimbra take advantage of a suite of open standards, including WebDAV, LDAP, iCal, and HTTP.

The significance of this open approach to messaging is remarkable. Because companies rely so heavily on e-mail as an essential business tool, critical enterprise data ends up stored on e-mail servers. When those servers are based on proprietary software, companies are at the mercy of a single vendor to orchestrate e-mail storage, retrieval, search, archiving, backup, integration, and management. A messaging system based on open protocols and open code gives customers assurance that they’ll be able to access their data when they need it, over the long term. Further, active development communities will increasingly allow those systems to compete with the proprietary stalwarts, not just on price and availability, but also on features.

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