Click on the email account you wish to modify and click on Edit within the menu bar.
Under "Sending Mail," click Click here for advanced server settings.
Check the box next to "SMTP server requires authentication."
Select the Log on using button.
In the Account ID field, enter the Yahoo! Member ID with the domain extension added. In place of the @ symbol enter a % symbol (ex MemberID%doman.net).
In the Password field enter the authentication password and click on the Save Passwordbox.
Click on the button in the top left corner of the window to save the configuration.
Click OK in the Edit Account window to save the configuration.
In the Accounts window, click on the button in the top left corner of the window.