Outlook.com is coming out of preview mode and is now available worldwide, the company said, so it will begin moving more than 300 million Hotmail users over to the new email service.
Hotmail users will get emails and other alerts informing them of the switchover to Outlook.com and its radically re-designed interface and organization tools, TIME magazine reported Wednesday.
Outlook.com currently has 60 million active members, Microsoft said, but when the transition is complete its hundreds of millions of users will put in on a par with Gmail's 425 million and Yahoo Mail's 281 million.
Hotmail users who want to retain their Hotmail.com email addresses can do so, the company said, and can simply transition to take advantage of Outlook.com's user interface and features.
Hotmail, originally MSN Hotmail, has been online since 1997.