I heard one can't use Yahoo's free email with POP clients like Outlook, Thunderbird or Apple Mail without upgrading to their premium service. At least this was the case last year when I tried accessing my Yahoo emails with Apple Mail and it simply refused.
Yesterday I tried it again, and now it works.
I have not purchased any Yahoo Premium Services. This is how my "premium services" look like:
All I did was to enter my Yahoo email ID and password when I opened Apple Mail for the first time, and used the "automatic setup" option. It did everything, no need to type in pop or smtp servers.
At first, I thought some software like yahoopops might be built-into OS X, or I may have installed one inadvertently. So I tried sending a mail, and it worked like a charm!
So now I have Yahoo's premium service for free. Do you?