Microsoft has taken its Outlook.com email service out of preview and opened it to the public, as it prepares to shut Hotmail down.
Squarely taking on Google's Gmail, the service already has 60 million users - making it the fastest-growing email service ever, says Microsoft.
All users of Hotmail - which Microsoft acquired in 1997 - will be switched over, although the company promises that nothing need change.
"The upgrade is seamless and instant for people who use Hotmail. Everything from their @hotmail.com email address, password, messages, folders, contacts, rules, vacation replies, etc. will stay the same, with no disruption in service," says David Law, director of product management for Outlook.com.
"When upgraded, they'll also get all the benefits from the redesigned Outlook.com experience - a fresh and intuitive user interface, lots of new features and better performance. And we won't ever make you switch your email address to an @outlook.com address if you don't want to."
As part of the launch, Microsoft's launching an aggressive marketing campaign costing tens of millions of dollars. It's pushing features such as Sweep, which allows messages to be moved, deleted or archived en masse.
It's also stressing the differences between Outlook and Gmail when it comes to advertising, echoing the message highlighted in its current Scroogled campaign.
"A number of people have expressed appreciation that Outlook.com replaces advertising with the latest updates from Facebook or Twitter when they're reading email from one of their contacts," says Law.
"In fact, on average, people saw 60 percent fewer ads when using Outlook.com because they're now getting much more relevant updates from their friends."
Microsoft says that all users should be switched over to the new service by this summer.