Thank you. That's a help.
I did not use gmail to send them. However, the lady who is assisting me does use Yahoo, and the account that she uses with me is forwarded through Yahoo because it was easier for her.
So I will assume that yahoo does the same as gmail. That is my working hypothesis, and I will check what's going on in the next mailing. I will make sure she does it from my own usual account in the future; and hope that fixes it.
Thank you.