The connection to the outgoing server "mail.btinternet.com” failed. It has been doing this all day. Shutting down and restarting doesn’t help; nor does deleting the account and recreating.
This might well be a “red herring”. The email I have been trying to send was to report a scam. After further checks, it could be that the outgoing server recognised the body as spam, so rejected it. If that is the case, please could I have a more informative message if the same thing happens again! Make that a “much more informative message”.