There's little you can do, and none of it involves Thunderbird. The message is a response either from your email service saying that your message can't be delivered to the recipient's servers, or from the recipient's servers saying that the message can't be delivered to the recipient's mailbox. There could be many different causes for this - the complete error message may provide more clues. But they are all server messages, beyond your control.

You can verify that the address is correct and still valid. You might be able to reach a tech support person at your email provider for more analysis.

Your answer makes sense, Thanks Pete

You might be lucky and it turned out that there were server issues that have since been fixed. It wouldn't hurt to wait a few days and try again.

Thanks Lin, I will wait and try Pete

Often those message come about because the email address you are sending to has a full account. We have not seen much of that for 20 years as free storage's have been growing into the gigabytes. It was common when an accounts were routinely limited 25Mb or less. (my first was 5mb)

I expect lots more with the recent trend in shrinking free storage's as folk simply can't cope with deleting their mail or archiving it, refuse to pay and stop getting mail delivered as a consequence.

Thanks for you input..... I will see if this is the case

Pete