From net..
Yahoo does not provide what are called 'read receipts'. If you want to use read receipts, you would have to subscribe to Yahoo Mail Plus and access your account through an email client, like Outlook or Thunderbird, that does use read receipts.
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If you are using Microsoft Outlook with Exchange Server, you can easily request read receipts and get notified when your email is opened by the recipient(s). As such a tracking feature in not available in web email like Gmail, Yahoo! Mail or the new Windows Live Hotmail, here’s an extremely effective and free solution that works with every email program...
rest..
http://www.labnol.org/internet/email/check-yahoo-gmail-email-read-receipts/2150/