I’m getting the following when I send an email. I can receive emails just fine:

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 'tax form'

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): Yes