Open source messaging is about to take a huge step forward with the integration of the Zimbra Collaboration Suite into a national offering from Comcast.

Comcast will leverage the Zimbra platform to offer its customers e-mail, instant messaging and voicemail within one online dashboard.

Financial terms of the deal were not disclosed. HP is involved in the deal as a systems integrator and will provide managed services.

"What Comcast has done is take the base Zimbra platform and then provide extension points for visual voicemail and for the integrated address book," John Robb, vice president of marketing and product management, told internetnews.com

Robb said the idea is to create a general purpose platform that carriers and ISPs can use to create a white label service for customers; the individual carrier can then differentiate the offering by building unique components on top of the stock Zimbra offering.

According to Robb, Zimbra had been in discussions with Comcast for a long time -- meeting with Comcast before Zimbra came out of stealth mode last year -- before consummating the deal.

Zimbra includes a collaboration server that handles the back end of e-mail, calendaring and collaboration, and a browser- based AJAX client front end, which runs on all major browsers.

The Comcast deployment could well be one of the largest ever for Zimbra, requiring the system to scale to a very high number of users. It's a challenge that Robb said Zimbra is ready for.

Robb explained that HP tested the Zimbra system at a higher volume than Zimbra itself had ever tested it. HP scaled the system to see just how high it could go. It turns out the core Zimbra architecture can handle the volume that Comcast needs, Robb said.

Robb noted that Zimbra is using standard MySQL for the Comcast implementation without any issues around availability or scalability.

Robb admitted however that Zimbra did have to make some adjustments.

"We did change the way we deal with users and data and inactive users in order to make some improvement," Robb said.

Improvements that come as the result of the Comcast collaboration may well end up in future versions of the core Zimbra platform. Currently, Zimbra is at version 4.5 with version 5 expected by the end of the year.

Zimbra recently issued a release claiming that it had crossed the six million paid mailbox threshold. Zimbra's six million paid mailboxes are spread across over 1,300 customer deployments.