This usually indicates a broken or corrupt address book or Mailing List.

Can you re-build your Mailing List (i.e. make a new version of it) and see if the new instance works?

Thanks Zenos for the suggestion. Though it did not provide a direct solution, it pointed me to one. Being loathe to try to reenter an address book with over 1,000 entries, I saved the existing book, and replaced it with a copy from a computer backup when I knew the email function was working as desired. I got the same problem. I tried exporting the book, to save what I could at least, and ran into problems I've since discovered came from trying to export "All Address books" I then opened a copy of the AB.MAB file in Quattro Pro, where I noted that QP divided the description section of the problem mail list into separate cells,, based on commas in that description. I returned to Address Book, removed the commas from the description, and my group e-mail went out as smoothly as could be desired. I'd note that the commas have been in that description likely for 15 years or more, and only now caused a problem. I assume that is due to some change in the latest Thunderbird update. It would be nice if we got warnings about that kind of change. Answer to my problem: Remove commas from the description in the list properties.