Outlook.com has seen more than 1 million people sign up for the service in just a few hours, Microsoft has said.
The sign-up figures for the successor to Hotmail were tweeted by @Outlook yesterday just over six hours after the new service was first announced.
The changes include the new e-mail address -- @outlook.com -- which will replace the older @hotmail.com address, used by hundreds of millions worldwide across its more than 10 years of its service. Users can already "upgrade to Outlook.com" to keep their existing address.
The new Web-based e-mail service will eventually integrate Skype, delivering a new rival to Gmail-embedded Google's Talk service. Microsoft acquired Skype for $8.5 billion last year after European regulators signed off on the deal.
But the real measure of success will be if existing Hotmail users voluntarily make the jump to Outlook.com of their own accord: the software giant still has to hope it can generate enough buzz around the new service to prevent a mass exodus to rival services, such as Google's Gmail.
Google remains in the lead with 425 million users, while Hotmail had more than 350 million at the last count.