The message could not be sent because the server rejected the sender's email address. the sender's e

Why do I get the following message in my live mail when trying to send an email
The message could not be sent because the server rejected the sender's email address. The sender's email address was '[email protected]'.

Subject 'is this the right place?'
Server Error: 550
Server Response: 550 5.1.0 Authentication required
Server: 'smtp.comcast.net'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 587
Secure(SSL): No