While Google allows you to read all your unread Gmail messages in any RSS reader that supports authentication, such a feature in not available in Yahoo Mail by default.
Am not sure if these RSS feeds for email can make anyone feel more productive but if you are on Yahoo! Mail and need to read your new email messages via an RSS feed, check out Ymail Feed.
Yahoo! Mail feed is a new Google App Engine powered service that lets you create an RSS feed of all the unread email items in the Yahoo! inbox. Unlike the Gmail RSS feed, feeds created by YMail Feed don’t require authentication so you can pretty much subscribe to them in any news aggregator including Google Reader or the Web clips in Gmail.
The authorization is handled by Yahoo! so you don’t have to share the password anywhere. A caveat though - mail feeds syndicated via YMail don’t require authentication and hence others too can subscribe to the feed provided they know the feed address.
You can however add the Yahoo Mail RSS feed to your Firefox live bookmarks and get new mail notifications directly in the browser. And there’s an option to reset the feeds in case you’re a little worried about security.