I need to know that a particular e-mail was received and opened. How do I tag the outgoing e-mail?

You don't. That's called a read receipt and it is not a service offered by Yahoo's free email. To get this, you have to subscribe to Mail Plus.

"lucee" is right that read receipts are not a feature of free Yahoo accounts. They are also not a feature of Mail Plus. Subscribing to Mail Plus does, though, allow you to use an external email client program to manage your emails, and it is in the external email program that you would find read receipts. The most common email clients are Outlook and Outlook Express.

By using "YPOPs!" you can get Outlook to work with Yahoo! mail regular and Yahoo! mail beta too. I checked it and it sounds really good! Note: even when using mail plus or any of the other mail progs, you do not have the guaranty that you get a read receipt because a recipient can opt out of sending such notices. Reason behind this: by denying a read receipt, you make sure that spammers don't know whether they hit a real mail address. http://ypopsemail.com/