Windows Live Mail authorising problem

I have been moving my Live Mail account from an old windows 7 pc to a new windows 10 pc. 

Everything seemed to have gone v well until I discovered I could no longer send emails. My account seems constantly to be authorizing and my emails get stuck in my outbox. 

Here is a copy of the message I have received:

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

Subject 'Fw: XXXXXXXXXXX
Server Error: 530
Server Response: 530 authentication required - Your email could not be sent. To fix this you must make a simple change to your email client (known as SMTP authentication). For advice visit http://www.bt.com/smtp
Server: 'mail.btinternet.com'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 25
Secure(SSL): No

Apologies I am not sure how to proceed