After going through my documentation on Postfix again, and trying to figure out why those problem mails keep resending to my system, I discovered that the error code I had set, 450, was basically telling the other end that there was a problem, and to try again. I've now changed this to 550, which should now reject those mails.
This seems to be good in that those sending me mail should not receive the error mails that indicate a problem mailing me, letting them know about the problem. I've begun sending postmasters of mail servers with issues notes containing clips from my logs in the hope that this might help them fix their systems. We'll see what happens.
This last error I saw from actually from what should have been a Mac system, so hopefully they'll get their act together quickly. ;)