We are thrilled to announce the open availability of the Yahoo! Mail Web Service, web service for Yahoo! Mail (accessible via SOAP or JSON-RPC) that we previewed to Yahoo! Hack Day attendees. With the Yahoo! Mail Web Service, you can connect to the core mail platform to perform typical mailbox tasks for premium users such as list messages and folders, and compose and send messages (you can also build mail preview tools for free users with limited Web Service functionality). In other words, developers outside of Yahoo! can now build mail tools or applications on the same infrastructure we use to build the highly-scaled Yahoo! Mail service that serves nearly 250 million Yahoo! Mail users today — users who might want to help you make some cash with your application.
More info: Yahoo! Developer Network