Keep in mind that you didn't reveal if you PAID for the POP3/SMTP Yahoo email service. If you are trying to use the free webmail via Outlook, it can fail since Yahoo made many changes over the last month. You get to set it up again per their instructions.

Also, if you use a firewall, I'm seeing Gmail, Yahoo, Hotmail and other fail because of the firewall settings. It has to do with items on the page that are not coming from the same server. I offer this as a clue and as no support of the firewall.