Email App Passwords Suddenly Stop Working

I have been using the Outlook email client to access my virgin media email for a long time. I have changed the app password using the 4-hyphenated password generated under the account details tab several times successfully. Over the last few days the password stopped working and my attempts at using a new app password have failed.

When I run the command line prompt:  curl -v smtps://smtp.virginmedia.com -u mailto:"username":"app password", I receive the response

235 2.7.0 SMTP350 Authentication successful for user...

The only other explanation could be that my IP address has been blocked by the server.  However I can access my emails via the Virgin media website.

So 2 questions:

1: Why does my app password spontaneously stop working?

2: How can I reconnect via the Outlook app, given that repeated app password changes are not working? 

Many thanks.