The best way to deal with this is to visit your email provider's website, log in to your email account there and deal with the errant message there. You'll probably need to delete it to release the others stuck behind it. If it's important and you want it in Thunderbird, forward it to yourself then delete the original.

It's probably misconfigured or corrupted in such a way that Thunderbird is left thinking there is more to come, so won't mark it has having been downloaded, and so it remains in the queue.

Rather as if I stopped writing without finishing the

Thanks , that did the trick !