Developers can now plug their software directly into Yahoo Mail, thanks to some new APIs.

Yahoo has delivered application programming interfaces for Yahoo Mail, said Chad Dickerson, head of the Yahoo Developer Network. The move comes on the heels of an announcement to offer unlimited storage capacity for Yahoo Mail users starting from May.

Officials had indicated in September their intention to let external developers write applications for Yahoo Mail, and the tools will now be available at the Yahoo Developer Network web site.

Yahoo provides similar development tools for other services, including the photo sharing site Flickr, the Yahoo search engine and the Yahoo Shopping product comparison site.

Companies such as Yahoo, eBay and Google believe they benefit when external developers come up with useful applications for their online services that their internal developers don't have the time or resources to create.

With about 250 million users, Yahoo Mail has a gigantic audience, and the company is confident external developers will be very interested in writing applications for it, Dickerson said.

"By opening up our APIs, we'll begin to tap into the world's ideas about mail," said John Kremer, Yahoo Mail's vice president.

As an incentive, Yahoo will pay a commission of $10 for every Yahoo Mail Plus account created as a result of a referral from an approved developer. Yahoo may come up with other monetary incentives and business models for developers, partners and independent software vendors.