Open source collaboration and messaging service Zimbra is banking on geek features to create popularity, but enterprise sales are still driven by straightforward business demands.

A quick demonstration of Zimbra is bound to delight the average software enthusiast. The application is essentially Outlook on steroids.

Street addresses inside an email automatically link to a Yahoo map, and phone numbers can be dialled simply by clicking on the number.

The software is free under an open source licence. About one year after its launch, Zimbra has signed up about six million users on commercial licences that include updates and support.

The company does not disclose revenues, but current sales are estimated to be just under $100m a year.

The advanced features and plug-ins, so-called Zimlets, helped the company to reel in customers like Digg and Mozilla, according to Zimbra chief technology officer Scott Dietzen.

As the software is available under an open source licence, developers can add additional functionalities as they please.

Online retailer Backcountry.com, for instance, has written a mini application that allows employees to check on inventory levels for products mentioned in emails, including order confirmations.

But Dietzen is quick to admit that the majority of the company's six million paying users are simply looking for an inexpensive service that offers email and calendaring and doesn't crash.

"To get the technology community behind you, you've got to have technology leaders using your product," Dietzen said at a meeting with reporters.

"People look at those early adopters for their information on what is cool and hip. But being sexy is only effective if you are stable."

Although Zimbra is an obvious competitor to Microsoft Exchange, it is rarely used to replace an installation of the latest Exchange server.

Most customers adopt the software to replace legacy email clients that lack calendaring services or a clear future roadmap, such as Sendmail or the Mac version of Microsoft Office.

In the last case, Microsoft has unlinked development roadmaps for the Windows and Mac version of the product, raising doubts about the future direction of Office on the Mac.

Zimbra is also profiting from Microsoft's lack of an email service for Linux systems.

The corporate email market today is ruled by Exchange, Novell's GroupWise and IBM's Lotus-Domino.

Dietzen said that he is confident that one day the company will be battling those vendors for the top spot.