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.

Late on Wednesday, Yahoo plans to deliver application programming interfaces (APIs) 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.

Yahoo officials had indicated in September of 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 Web site.