Temporary Error 21
Occasionally our users experience temporary, transient issues in Yahoo! Mail, and the error message "Temporary Error 21" indicates one of these. We apologize for any inconvenience that this may cause. Our engineers have already been notified that an error has occurred and are looking into it. We have found that most of these issues are resolved in a few minutes to a few hours. Please try again in a few hours and your mail should be available.