yahoo of late appears to be insisting that BCC: mail must have a valid to: address as well. Are they your provider?

Yes, ATT mail uses Yahoo, so I sent myself a blind copy at an alternate address, and it worked. This must be a recent move by Yahoo, as I had not had a problem before. Yahoo needs to let folks know about this.

Yahoo needs to let folks know about this.

They do, But Yahoo are not loosing customers in the US at rates high enough to alarm them. So I guess their current lackluster communications and arbitrary decision making is acceptable to the US market.

I can not find any reference to it anywhere on their site to a change, just customers with something that does not work trying different things. Perpetuating the myth that email is some sort of lottery and settings are guess work. With a good mail provider nothing is guesswork because they document their policies and settings.