Receiving mail just fine, but sending mail produces error message:

Subject 'Thank you'

Server Error: 530

Server Response: 530 Authentication is required before sending [R0107005]

Server: 'SMTP-server.nc.rr.com'

Windows Live Mail Error ID: 0x800CCC78

Protocol: SMTP

Port: 25

Secure(SSL): No

What is "530 Authentication" and how do I supply it?