Reverend Ted and all:
Well, here's my bizarre workaround. Out of curiosity, I opened an E-mail account with Gmail, which uses smtp server settings of smtp.gmail.com port 465. I used those settings in the DNR-202L event page. I had no success with a test e-mail on Port 465. So I tried Port 25, was able to get a successful Test E-mail, as well as successful event E-mails, including motion and video loss. So, it must be some kind of ATT Yahoo quirk preventing the event e-mails.
So, Gmail works - on port 25.
I can't really explain it all, but it works.
Thanks again!
Steve