I suspect you have hit it on the head. Perhaps my shock at the 3 hour drop window is because I have 20+ years of email experience (including bang addressed uuc

I suspect you have hit it on the head. Perhaps my shock at the 3 hour drop window is because I have 20+ years of email experience (including bang addressed uucp mail) and the prior norm was an undeliverable but will retry until XXX message, not just a we give up message.