The most common reason for receiving Yahoo Error 999 is due to some sort of bandwidth limiting system that Yahoo has put in place on their servers. Once you have exceeded your allotted bandwidth for a specific period of time Yahoo gives you this Error 999 message and doesn't allow you to access the service. People have primarily reported receiving this error when they try to access Yahoo Mail or Yahoo Groups, but other Yahoo services may also be affected.
Why has Yahoo done this? There are two reasons that I can think of:
1. To prevent DoS (Denial of Service) attacks.
2. To stop automated tasks from hammering their servers with hundreds of requests a second.
it works now it had to be yahoo problem but I something is still wrong. I run one domain name through it it comes up with 5700 backlinks i click on show and it only shows 1000.
Thats correct, Yahoo only returns 1000 backlinks max.
The 999 - unable to process request at this time is a self defense mechanism implemented in many Yahoo! services and web servers as a way to prevent abuse. The 999 error is actually defined as an HTTP response code (http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html) with the value of 999 from a Yahoo! web server. The 999 response code is usually accompanied with an HTML body text saying that the service is unavailable but this is not always guaranteed to be the case. The 999 HTTP code is a custom HTTP code specific to Yahoo! web servers.
The backlink checker addon works just fine.
I just checked without proxies, and with private proxies, no error.
If you get a 999, it might be a temporary yahoo issue or your proxies are bad.