Today, Microsoft's MSN unit expanded the beta program for its upcoming Hotmail replacement, currently dubbed "Mail Beta" and codenamed "Kahuna," so that a much wider range of users can test the service. Kahuna combines the look, feel, and functionality of true email clients with the portability users have come to expect from Web-based email solutions.

"There's been a rush of activity in the Web mail space over the past 18 months, " Kevin Doerr, who oversees Hotmail and new Web mail experiences at MSN, told me during a briefing last week, "and an explosion in the way people use email, especially around sharing information through attachments and digital photos." Kahuna seeks to address these changes, and it comes at a propitious time for the company: Hotmail, the service it will replace, is 10 years old now and is architecturally strained at its current traffic rates. "When we purchased Hotmail 9 years ago, it had 8 million users," Doerr told me. "Today, there are over 200m accounts worldwide, and a big percentage of those active are active."

Kahuna has been architected from the ground up to handle the massive amount of activity the service will generate, address competitive issues, and provide users with an elegant user experience that goes far beyond the typical Web mail solution. Behind the scenes, Kahuna uses a new set of technologies called AJAX that combines Javascript, Dynamic HTML (DHTML) and other Web programming features to create advanced user interfaces that can often rival what's available to true Windows applications. Thus, Kahuna supports advanced functionality such as drag and drop, keyboard shortcuts, and right-click menus.

Until today, Kahuna had been in its first phase of development, in which a small group of 5000 testers--made up largely of Microsoft employees and experienced MSN Messenger beta testers--put an early version of the system through its paces. Today, MSN launches phase two of Kahuna development. The solution is being opened up to 200,000 testers and is adding a number of new features and functionality over time; for example, the calendar component was recently added back to the core Kahuna feature set. During this phase, Microsoft will also open up Kahuna to international markets as it adds localization support.

Finally, in phase three, Kahuna will be scaled up to the hundreds of millions of users that Microsoft expects will eventually use the service. Performance and reliability will improve, and fit and finish issues will be ironed out. Microsoft expects to ship the service live in the first half of 2006.

As for branding, Microsoft says it will figure that out later. Right now, they'd prefer people to call it Mail Beta and not Hotmail. I'll have an extensive preview of Kahuna available on the SuperSite for Windows  late today, but if you're a US consumer interested in signing up for the beta program, please visit the Microsoft Web site.