Generally you only see this sort of error when the recipient and sender are on the same server. However, it may be your setup is actually contacting the to check the recipient address. It would be most unusual, but not impossible. The issue here is did the email you sent via google actually arrive? That would indicate at least if there is something odd in the SMTP server, or if it is actually just a wrong rsponse.

Yes, my Gmail response went through and they confirmed receipt. So, either Thunderbird or my ISP email server is doing something strange.

I contacted ISP customer service, but they were baffled, as am I.

Next week, when my attorneys are back in the office, I will try with a different email program, just to see what happens.

I don't want to stop using Thunderbird, tho. I like it.