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.
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.
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