Thank you, Thomas. Appreciate your thoughts. Helps rule out several possibilities. As I had no reason, nor any inclination to root around in esoteric email settings, another possible suspect for the new behavior is the macOS update. Casual searching so far has not found any other reports of issues like mine. Finally, and perhaps more likely, as a user of AT&T email, I have seen their struggle to stay afloat. The telling sign is seen in the URL now used: "currently.att.yahoo.com"! Looks more like a floating crap game than a stable operation.
Thanks for your prompt and thorough reply.
Doug