The only developers usually allowed access to Yahoo Mail code are from its broadband business partners at BT and AT&T.
Yahoo will maintain absolute control of usernames and passwords to ensure security, but developers will be able to create new interfaces and manipulate email content, as well as design 'mash-ups' that link to other web services.
These could include apps that link to Google Maps, Flickr photos, and rival email programs Gmail and Hotmail.
Chad Dickerson, head of Yahoo's software developer relations programme, told Reuters: "Yahoo is a very large company but we cannot build every application that a user might want.
"You can imagine tens of thousands of niche applications [springing] from Yahoo Mail."
Yahoo Mail is considered the most popular consumer email service available on the internet, and is used by an estimated 257 million users globally.
_(1).jpg&h=140&w=231&c=1&s=0)
