My understanding of this answer is that the SMTP said it had accepted it, but the sender would have been notified afterwards that the message had not been deliv

My understanding of this answer is that the SMTP said it had accepted it, but the sender would have been notified afterwards that the message had not been delivered, had the return addresses correctly been set up. This seems OK to me; I would have received the bounce. (Also, the mail was sent from www-data, which I think was an acceptable clue to classify the message as spam no matter from which server it came from.)