I want to follow up again, about one thing here.
It is extremely unhelpful, but you can thank Gmail for that banner of misinformation.
If you scroll down and read the fine print at the bottom, there it does say you are not subscribed.
That's the actual response given by Groups.io when Gmail attempted that message delivery.
But why does something (Google?) insert that other, large, unhelpful message? Can anything be done to eliminate the "misconfigured" part?
I'm sure Mark wishes he knew answers to those questions. In particular if there's some other response code that would give a more sensible answer, or if Gmail is reading the response text for some keyword. I may be giving Gmail too much credit, they might be just ham-handed about these things.
As a Gmail user myself I've wondered about some of their recent, more "panicky" reactions to various email situations.