Starting with Yahoo! Toolbar 6.2 for Internet Explorer, there is no longer a separate mail notification icon on the toolbar.

When you receive new Yahoo! Mail, the icon on the Mail button updates to show you the number of new mail messages available right on the button, like this:

Note: You must be signed in to Yahoo! and must have the Mail button on your toolbar in order to receive new Yahoo! Mail alerts.

Click here to add the Yahoo! Mail button to your toolbar (sign in required) if you don't already have it.