Google declined to comment on Outlook.com. The company, which is based in Mountain View, Calif., plans to convert all of its Gmail users to its redesigned format within the next few months.
Yahoo, which is based in Sunnyvale, Calif. revamped its email service late last year in an effort to provide a more consistent experience on personal computers and mobile devices.
Outlook.com is the latest in a series of major product leases from Microsoft, which has been struggling to regain the cachet that once made it the world's most valuable technology company.
Now, both Apple Inc. and Google are worth more because they have been growing far faster than Microsoft as their products win more fans. Apple's biggest gains have come from the iPhone and IPad, while Google has been benefiting from its dominance in Internet search and its widely used Android software for mobile devices.
Microsoft has been trying to catch up with a major makeover of its Windows operating system, new smartphone software and a tablet computer called Surface. Like Outlook.com, all those products have been backed by expensive marketing campaigns in recent months.
Investors evidently aren't convinced there will be a big payoff. Microsoft's stock increased 26 cents Tuesday to close at $28.05. That's just 20 cents above the stock's price in late October when Microsoft released Windows 8. Google's stock has surged by 19 percent during the same stretch.
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