I think you are using the Free yahoo account (just a guess).
Yahoo (free version) does not include the POP3 feature to be used with any email client like Outlook, thunderbird, Windows Live Mail etc... If you are using the free email from Yahoo, you will need upgrade it to the Yahoo Mail Plus. It is $19.99 per year. Check with Yahoo Customer for the upgrade else, login to your Yahoo account, go to Mail Options. Click on the POP and email forwarding option. You will find the option to upgrade to the Yahoo mail Plus.[/
Thanks for the input!