When I attempt to send an email, it goes to the outbox and sits there. When I attempt to send it again, I get a bit of a delay and then an error message that looks like this:

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 'Re: How was your day?'
Server Error: 550
Server Response: 550 5.7.1 <*** Email address is removed for privacy ***>... Too many invalid recipients.
Server: 'smtp.sasktel.net'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 587
Secure(SSL): No

What is wrong? What does Error 550 mean?

I have Googled this problem and tried quite a few suggestions with no luck. I already tried the Windows Essentials repair in the Control Panel.