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The ability to access Yahoo! Mail via a POP3 email client is only available to customers of Yahoo! Mail Plus. If you have not purchased the Yahoo! Mail Plus service, you will be unable to retrieve messages via an email client (such as Outlook or Outlook Express). You can determine whether or not you've purchased this service by visiting the password-protected service summary at this link.
If you have Yahoo! Mail Plus, make sure you have the correct password entered in your email client. To test this, log in to your Yahoo! Mail Plus account. Enter your Yahoo! ID and password. If you receive an "Invalid Password" error, then you'll need to change your password. When you change your password, return to the email client and make the appropriate changes.