I've seen a similar attack in my inbox recently where people do the same trick by sending an email to yourgoogleaccount@google.com instead of Gmail.com and then

I've seen a similar attack in my inbox recently where people do the same trick by sending an email to yourgoogleaccount@google.com instead of Gmail.com and then forwarding the bounce from Google's mail servers to yourgoogleaccount@gmail.com with a spam message smuggled at the end. Passes all checks. Just kinda weird that a postmaster error is intertwined with a phishing campaign. But easy to gloss over as a non-technical user. In my case the phishing campaign told me I won some construction tools or something.