|What browser are you using? |
If it's Mozilla, go to Tool >options, Advance tab
Under Advance tab, you will see a Network tab, click on settings, check to make sure it's set to "No proxy"...
Mostly when this settings is set to "Manual Proxy configuration" , you get this 999 error when accessing Yahoo Mail on the web but some sites would still be working under a proxy ip set in the manual proxy configuration..
If you are using IE, go to Tools > internet options > connection tab, LAN settings? then untick the box that says: Use a proxy server for your LAN ( These settings will not apply to dial-up and VPN connections).
You can check the box "Autromatically detect settings" if you want to.
CLick on OK > Apply > OK button, then go to www.mail.yahoo.com, sign in and see if you can go to your Yahoo Mail.