NEW YORK (CNNMoney) -- Microsoft has decided to radically overhaul the most successful email service in the world. Goodbye Hotmail.com; say hello to Outlook.com.

Over the next few months, a growing number of Hotmail users will be invited to test out a new Web-based email service called "Outlook," named after Microsoft's other popular email brand. Eventually, everyone will be transitioned to Outlook, but Microsoft gave no firm deadline for that to happen.

Hotmail users will be able to keep their @hotmail.com or @live.com addresses, but new users will only be able to choose between @outlook.com or @live.com email domains.

"We thought it was time to push the boundaries of email," said Chris Jones, head of Microsoft's Windows Live division, which includes Hotmail. "We created a new mail service for a modern user experience."

Hotmail, launched in 1996, has just over 325 million users around the globe. That makes it the Web's largest email service, but its lead is shrinking. Google's Gmail now has 298.2 million users. Yahoo Mail is right behind, with 298 million users, according to tracking firm comScore.

Hotmail's newer rivals have blown past its aging feature set.

When it comes to social integration, labels, filters, and user interface, Gmail -- and Yahoo, to a lesser degree -- offer more complete services. The name "Hotmail" even sounds like a relic of the 1990s, in the same way that having an AOL (AOL) email address has become a punch line.

That's why Microsoft (MSFT, Fortune 500) decided to start over.

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