✔ 最佳答案
Yahoo's error 999 is usually caused by a bandwidth limit or possibly by any activity that looks suspicious from your IP address or proxy. This is a temporary problem and should fix itself if you just wait a little while before signing back on. If it doesn't, here's a link with some extra tips to try as well as more info on this error code:
http://www.murraymoffatt.com/software-problem-0011.html