Did you setup your email account correctly?

Instructions below


Incoming Mail (POP3) Server: pop.mail.yahoo.com

Outgoing Mail (SMTP) Server: smtp.mail.yahoo.com

(use authentication, optionally use port 587)

Account Name/Login Name: your Yahoo! Mail ID (your email address without the "@yahoo.com")

Email Address: your Yahoo! Mail address (e.g. user@yahoo.com)

Password: your Yahoo! Mail password

Please note that if your client does not support SMTP authentication, you will not be able to use it to access your Yahoo! Mail messages.

Make sure that you change the port numbers. Using Thunderbird with the wrong port numbers will cause it not to function.

Hope this helps.