I'm fascinated that someone would want to use a brute forcer to retrieve a password rather than one of the many other options provided by yahoo. When you sign up for an email account you answer security questions and specify a secondary email address.

More likely you're trying to get into someone else's email? In that case, look up the Sarah Palin email hack. You'll get more mileage with that method. Nihil was kind enough to explain why a brute forcer would not apply in this situation.