Two new versions of open source software aimed squarely at Microsoft's Exchange server have been released.

Scalix announced version 11 of its rival to Microsoft's ubiquitous email software, adding features such as real-time indexing of email for fast searching.

Meanwhile, Zimbra released version 4.0 of its Zimbra Collaboration Suite with a new tool that lets users embed fully functional documents and spreadsheets in email.

Microsoft itself plans to release Exchange 2007 early next year, but it will work only on systems running 64-bit versions of Windows Server 2003. That could provide an opening for vendors, said researcher Michael Osterman.

Scalix offers a community edition of its software that users can try for free, and its commercial release lets companies move to a browser-based email interface that uses AJAX technology to provide features similar to those built into desktop applications. It also allows users to continue running their existing Outlook clients.

Zimbra, which launched the first version of its software last October, offers the same browser and Outlook options that Scalix does, plus contact management and calendaring features. But Osterman warned that users who need features beyond the groupware capabilities offered by Scalix, Zimbra or Open-Xchange may find themselves saving less money than they expect by installing one of the open source technologies.

"If you need integration with Office or Live Communications Server, or a document repository, or you have a very distributed organisation," he said, "you could lose the cost advantages you were seeking."