Yahoo! Mail Web Service User Guide and API Reference

You must use the Browser Based Authentication API to enable access to the user's Yahoo! account through your Application.
You may not use the Yahoo! Mail Web Service API to mine or scrape user data from the user's Yahoo! account.
You may not use the Yahoo! Mail Web Service API to recreate a mimicked version of Yahoo! Mail on your site.
You may not use the Yahoo! Mail Web Service API to display the user's Yahoo! account information in a third party email client.

Abstract

The Yahoo! Mail Web Service gives developers access to APIs for Yahoo! Mail. With the Yahoo! Mail Web Service APIs, you can build Web applications to perform tasks such as listing messages, displaying folders, and composing and sending messages.