I cannot send e-mails from Thunderbird to ...

What prevents you from sending messages to this email address?

They just don't arrive! But they do if I send them from my BT Account.

Is a copy of the message in your Sent folder? Do you get any error message?

No, no error message and a copy appears in my sent folder

A copy of the message in the Sent folder indicates that the message has been sent. So the problem is not with your Thunderbird.

There could be a problem with your email provider, with the recipients email provider, or with the recipients email client. I'd check with the latter first, e.g. whether the message was classified as spam.

Thanks for your help but haven't solved it yet.

In order for us to better investigate this issue, we'll need some more information.

Could you please do the following:

1. Open the menu > Help > Troubleshooting Information, then click Copy text to Clipboard.
2. Open a reply to this post, and paste in the troubleshooting information.

All that is really required of that information is down to the heading Graphics if you have issues about length.