You're reading way too much into my request. In NO WAY do I suggest monitoring private messages. There's no harassment to letting someone know the contact information has bounced. Email clients do this all the time.
It would be nice to know if my post was delivered that way I would know that the person is no longer interested in communication. But my main interest is knowing if the contact information is still good. People die, lose interest, or simply neglect to update contact info.
All I am really suggesting is keeping a person informed if the address BOUNCES. Email clients do this routinely. It's nice to know if the contact information isn't any good.
All staff really has to do is disabled the account. I'm sure you have seen accounts with no link under the user's name? These are disabled accounts. It would be better to not be able to privately contact someone than to waste time sending info and not knowing if it was received.