I cannot send emails using Windows live mail. The message I keep on receiving is the following: 

Server error: 550

Answer from server: 550 5.7.3 Requested action aborted; user not authenticated

Server: 'smtp.live.com'

Error ID Windows Live Mail: 0x800CCC78

Protocol: SMTP

Port: 25

Secure(SSL): Sim

Can you please tell me how to fix this? Many thanks.