If revenge is a dish best served very cold, and revenge is also sweet....is revenge ice cream?
(yes, I shamelessly nicked that from a philosoraptor meme)
Anyway, I've been asked to set up similar 'no bounce' systems, and I've simply told the client - in professional terms - that it's a staff competency problem, not an IT problem.
If they aren't capable of using the address book of their email clients, then they probably shouldn't be trusted to handle a spoon without adult supervision due to the risk of injury.
In this case? I'd have done the same. If the Big Boss wants me to compromise my professional ethics by intercepting staff email on the fly, they can go fuck themselves.