Microsoft Exchange can sometimes bounce a message without providing any indication of the address to which the original message was sent. When Exchange knows the intended recipient, but is not willing to accept email for them, it omits their address. If a message is sent to codice_1 and the server knows that this is "Joe User", it will bounce the message saying that the message to "Joe User" could not be delivered, leaving out the codice_1 address altogether. VERP is the only viable way to handle such bounces correctly.