can not send out emails. server is rejected senders email address and comes up with error code 530

I am setting up a new computer with window live mail and cannot send out emails I get the following message every time:

The message could not be sent because the server rejected the sender's email address. The sender's email address was (removed)

Subject 'Fw: hahaha!!'

Server Error: 530

Server Response: 530 authentication required - for help go to http://help.yahoo.com/help/us/mail/pop/pop-11.html

Server: 'plus.smtp.mail.yahoo.com'

Windows Live Mail Error ID: 0x800CCC78

Protocol: SMTP

Port: 25

Secure(SSL): No

Question about Outlook Express