The obvious solution is to just bounce invalid recipients, like everybody else
This right here, is how email works. User gets bounce, user realises their mistake and everybody is happy.
In this completely made up story the admin thought outside the box without reading any RFCs (as admins are prone to do) thought he was being clever and in the end not only snooped on other people's private lives (I mean using corp email for this stuff is begging for trouble but it's still essentially private) and also broke the entire global email system in the process.
This stuff doesn't have to be difficult but people really so enjoy trying to make it so.
Also if your boss has a problem with your email system working correctly go find yourself a new job and do it now because your company is probably going to go down in lawsuit and you're probably going to end up named as at best a witness and it's going to cost you a lot of money in legal fees.