Microsoft has officially closed the public-testing period for Windows Live Hotmail. Now begins the lengthy process of migrating users to this major upgrade of Microsoft's Hotmail webmail service, Microsoft will announce today.

Windows Live Hotmail is the biggest Hotmail upgrade since the popular web email site was first released in 1996.

Later this month, Microsoft will release a free software for linking Windows Live Hotmail with Outlook, a capability the company previously charged for. With Microsoft Office Outlook Connector for MSN, users of the Webmail service will be able to access their account from Outlook, along with email, contacts and folder synchronisation for free. In the future, Microsoft will release a desktop client for Windows Live Hotmail called Windows Live Mail.

In terms of security improvements to Windows Live Hotmail, each email message carries a "safety bar" which indicates whether the message came from a known, unknown or potentially fraudulent sender. Spam protection has also been enhanced. Windows Live Hotmail also has a mobile version so that its estimated 280 million users can access the service from mobile phones and handheld devices with a web browser.

Windows Live Hotmail has been tested by about 20 million users since Microsoft first allowed people outside Microsoft to try it out in mid-2005, according Brooke Richardson, Microsoft group product manager for Hotmail.

"It is safer, faster and has more features than Hotmail. It's definitely an advance for our current customers," she said.

Windows Live Hotmail has a new code foundation which will let Microsoft add improvements quickly, something that had become a challenge with Hotmail's decade-old code base, Richardson said.

"It's a much more stable foundation for us to innovate," she said.

New users will be automatically signed up for Windows Live Hotmail but, like any user of the new service, they will get to choose from two user interfaces: a "classic" layout that closely resembles the old Hotmail; or the new interface, which was designed to look like Microsoft's Outlook email and calendaring desktop application, she said.

Microsoft will not force Hotmail users to migrate Windows Live Hotmail. For now, Microsoft will give Hotmail users the option to continue using the old version if they don't want to switch to Windows Live Hotmail.

However, according to Richardson, at some point everyone will be unilaterally migrated from Hotmail to Windows Live Hotmail.