"Starting today, Microsoft will begin to upgrade every Hotmail user to Outlook.com.," the company said in a statement. "The upgrade is seamless and instant for Hotmail customers; everything including their @hotmail.com email address, password, contacts, etc., will stay the same."

Microsoft said last year it was overhauling its email service as it adapts for mobile users and social media.

The number of active accounts on Outlook.com grew to 60 million in just six months during its preview period, Microsoft said, adding it lets users connect to Facebook, LinkedIn and Twitter