Why am I gettin an error999 when trying to use my address book to forward e-mails?

Answer 1

After a little research via Google it would seem that Yahoo has blocked your IP address. The reason being they suspect it of being a pirated IP.
It also limits automated software like e-mail retrievers and web crawlers. Remember, bandwidth costs money. Yahoo uses a tremendous amount to service it's customers and must try to keep it within reasonable limits.
A lot of scumballs use IP (Internet Protocol) addresses to spam servers and create and DoS (Denial of Service) attacks just for a few examples. They steal the IP addresses of home and small business owners to do this.

Follow the link in the sources box for specific info.