Non-delivery reports (NDRs) are enabled by default. You can disable them by clearing the Allow non-delivery reports check box on the Advanced tab of the Default properties of the Internet Message Format, which is located under Global Settings in Exchange System Manager.
From The Hitchhiker's Guide,
"But the plans were on display..."
"On display? I eventually had to go down to the cellar to find them."
"That's the display department."
"With a flashlight."
"Ah, well, the lights had probably gone."
"So had the stairs."
"But look, you found the notice, didn't you?"
"Yes," said Arthur, "yes I did. It was on display on the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying 'Beware of the Leopard.'"