It was a wrong address and it's been fixed, but I can't clear the message.  It keeps repeating.  I'm running under Windows 7.  Both the Sent and Outbox folders are empty.  I've checked the mail account settings and made sure that the outgoing server requires authentication.  What am I missing?

Here's the message (I've blocked out the original address):

The message could not be sent. The setting for your outgoing email [SMTP] server might need to be configured. To find the server settings for *** Email address is removed for privacy ***', please contact your email service provider.

Subject 'hi'
Server Error: 550
Server Response: 550 5.1.1 Not our Customer
Server: 'smtp.comcast.net'
Windows Live Mail Error ID: 0x800CCC79
Protocol: SMTP
Port: 587
Secure(SSL): No