Reconciling this would unfortunately be a gnarly effort for comparatively little gain.
I’m certainly biased–I wrote the fast-rejection code that was removed–but I would disagree about the value of the concept, especially if the backscatter starts landing people’s Discourse servers on spam blocklists, or services like Mailgun start complaining about it (or worse, they don’t complain about it and happily charge you to send tons of bogus emails when a big wave of spammers pound through).
If someone else were to contribute code to resolve the concerns (parse the headers instead of the envelope, etc), would this be something Discourse would be willing to reincorporate, or is this just something no one has time to mess around with in any capacity at the moment?
(Either way is fine, I totally get that everyone is doing the best they can with limited time and resources!)