I changed from POP server setting  to iMAP as service provider required it. I can now receive messages but not send them

I now get the following message: 

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

Subject: 'Final status report"

Server error: 550

Server response: 550 5.1.0 Authentication required

Windows live mil error ID: 0x800CCC78

Protocol: SMTP

Port 465

Secure(SSL): Yes