And so I, and others on this forum, have some Idea why Thunderbird is as you say is refusing, perhaps you could share the error message being presented.

"There is already a file with this name in the same location" There are additional options like delete and add, or overwrite existing email; however just as I was preparing to send this, I tried again (for the 8th time) and I did not get the same response, in fact Thunderbird DID accept the new email and added it to my file. So, at this point there is nothing to do. Thank you for your response.