How 2 send receive emails in Outlook 2007 from my yahoo account ?

How to send receive emails in Outlook 2007 from my yahoo account ? I have installed Ypops and configured it and first time it downloaded my emails and i sent emails and it was working fine but now suddenly it has stopped working. Outlook give the following error when sending & recv email from yahoo account:
“Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP). The server responded: 554 Mailbox unavailable.”
some other errors
1. “0x800ccc90 avg pop3 server cannot connect.
2. 0x80042108
3. 0x80042109
and I have tried lots of troubleshooting but have not got any solution. Pls assist.