Zimbra, developer of an open source alternative for Outlook and Exchange, has signed up the largest cable operator in the US Comcast for its online messaging client.

The software will be used as part of a new communication centre that offers cable, broadband and VoIP subscribers access to voicemail, email and instant messages through a single website.

The service also partnered with Plaxo, an online address book and calendaring service.

The deal marks one of the largest agreements that Zimbra signed to date.

"This is a pretty significant deal for us," Zimbra's chief executive and co-founder Satish Dharmaraj told vnunet.com. "It's a defining moment for us as a company. We're not a startup anymore. We're doing some bigger deals and getting more mature."

Zimbra first launch its service in September 2005. It has signed up about 6m users for its commerical service. The privately held company doesn't disclose revenues, but estimates peg sales at less than $100m.

The Zimbra client software can be described as a more advanced version of Microsoft Outlook. The application automatically recognized data such as phone numbers and street addresses, allowing users to pull up maps and place a phone call with a single click.

The software is available to both enterprises as well as service providers such as Comcast. The desktop application is governed by an open source license, as is most of the server software. Several of the server's advanced features such as clustering and storage management however fall under a proprietary and require regular software license fees.

The open source nature of the platform ensures that developers can create so-called Zimlets, mini applications that add additional functionalities. Comcast for instance could quite easily add a feature that lets users select shows that they wish to record on their digital video recorders.

"That is well within the reach of [what can be done] today. Because most of the DVRs are web services powered and Zimbra is 100 per cent web services powered. So it's just a matter of putting it all together," Dharmaraj said.