Have you set up an app specific password which you use in Thunderbird to get connection to server? It seems to be a requirement.

https://www.virginmedia.com/help/broadband/manage-email-account Section called How to set up my Virgin Media email on a device Sub section : Setting your app password to use an Email app. If you want to access your Virgin Media emails from an app, you’ll need to create a separate app password through Account settings on My Virgin Media.

Once you have app specific password you need to modify saved passwords for both incoming imap and outgoing smtp:

• Settings > Privacy & Security
• Scroll to Passwords section
• click on 'Saved Passwords'
• click on 'Show Passwords'

locate each line for the account - right click on line and select 'Edit Password' completely remove all content and then type in the 'app password' you created. You need to do it for the incoming imap line and the outgoing smtp line. Then restart Thunderbird.

Hi Toad-Hall,

 Thanks for taking the time to reply. The problem has now gone away. It seems that the problem was with Virgin Media rather than at my end. Thanks again.