Yahoo is opening up its web mail platform to external developers, so that they can create plug-ins, utilities and applications for the popular Yahoo Mail service.

The move comes on the eels of an announcement to offer unlimited storage capacity for Yahoo Mail users starting from May.

Yahoo officials had indicated in September last year their intention to let external developers write applications for Yahoo Mail, and the tools will now be available at the Yahoo Developer Network website.

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

Rivals like Google also offer tools to third-party developers to build applications for their sites and online services. Companies like Yahoo, eBay and Google believe that it’s to their benefit if external developers come up with useful and interesting applications for their online services that their internal developers don't have the time or resources to create.

With about 250 million users, Yahoo Mailhas a huge 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 for developers to use the Yahoo Mail Web Service to create applications, 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.