For its 10th anniversary, Yahoo Mail is upping the ante to maintain its position as the leading free e-mail service in the world. It's bet: infinity.

"We will begin offering everyone unlimited e-mail storage starting in May 2007," said John Kremer, VP of Yahoo Mail, in a blog post on Tuesday.

When it launched in 1997, Yahoo Mail offered users 4 Mbytes of storage. Then came the information explosion. In 2004, storage capacity was raised to 100 Mbytes. A year later, it was 1 Gbyte.

Google's Gmail currently offers 2.8 Gbytes of storage to its users, an amount that has been creeping upward since the service debuted with 1 Gbyte. Google watchers speculate that some sort of competitive answer to Yahoo's announcement may come on April 1st, the third anniversary of Gmail.

Windows Live Mail offers 2 Gbytes of storage. MSN Hotmail offers 1 Gbyte for its free plan.

Yahoo Mail has long been the most popular free e-mail service worldwide. In February, it received some 243.4 million visitors, according to comScore Media Metrix. Microsoft's MSN Hotmail and Windows Live came in a close second, with 233.3 million visitors the same month. Google's Gmail remains a distant third, having served 62.4 million visitors last month.

However, of the three leading free e-mail services, Yahoo Mail has been growing the slowest. Between February 2006 and February 2007, worldwide traffic at Yahoo Mail grew 9%. Hotmail/Windows Live Mail grew 13% and Gmail grew 68% during the same period, according to comScore.

Unlimited storage, if unanswered by Microsoft and Google, may help Yahoo maintain its lead in the market.

"We hope we're setting a precedent for the future," said Kremer. "Someday, can you imagine a hard drive that you can never fill? Never having to empty your photo card on your camera to get space back? Enough storage to fit the world's music, and then some, on your iPod? Sounds like a future without limits."

It may sound like that, but limitless things often have limits, as is the case with Comcast's "All You Can Eat" broadband service. Kremer, in fact, acknowledged there will be some limits: "[L]ike any responsible Web mail service, we have anti-abuse limits in place to protect our users."

A Yahoo spokesperson declined to elaborate on limits to unlimited storage. "We do have controls in place to ensure that people benefit from the unlimited storage feature, but are not able to abuse the system," a Yahoo spokesperson said in an e-mail. "Our anti-abuse limits are there to monitor suspicious activity and to ensure our users have a safe, efficient and reliable Web mail experience. As always, we will require users to abide by the Yahoo Terms of Service. I do not have any other details to share with you at this time."