It's yahoo, so that might be the best you can do.
Is that the invalid username/password message? Did it ever work, and if so, did it stop working in mid-December? Yahoo had some updates about that time.
From
http://slipstick.com/addins/services/online-services-in-outlook/#yahoo - the POP3 settings are
Incoming Mail Server: pop.mail.yahoo.com
Outgoing Mail Server: smtp.mail.yahoo.com
User Name: Your Yahoo ID. You may need to use your full Yahoo email address.
Password: Your Yahoo password.
DO NOT enable Require logon using Secure Password Authentication
Click More Settings.
On the Outgoing Server tab, select My outgoing server (SMTP) requires authentication box and Use same settings as my incoming mail server.
Configure the server ports on the Advanced tab.
Incoming server (POP3) port: 995
Select This server requires an encrypted connection (SSL)
Outgoing server (SMTP) port: 465, 587, or 25
Set the encryption type to SSL or TLS