I rec'd this error message when I tried sending an email with a photo attachment:

The message could not be sent because the server rejected the sender's email address. The sender's email address was '*** Email address is removed for privacy ***'.

Subject 'Emailing: 8-27-11 464'
Server Error: 530
Server Response: 530 5.7.1 Authentication required
Server: 'outbound.att.net'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 465
Secure(SSL): Yes

How do I fix this issue?

Thanks!