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The designation that refers to mail that cannot be delivered is "Undeliverable mail." This term specifically refers to any mail item that cannot be delivered to the intended recipient for a variety of reasons, such as incorrect addressing, refusal by the recipient to accept the delivery, or absence of a known recipient at the address.

In postal operations, identifying undeliverable mail is crucial as it ensures that the mail can be processed accordingly, whether that means returning it to the sender, holding it for further instructions, or ultimately disposing of it if no action can be taken. Understanding this terminology is important for proper mail handling and ensuring that postal services maintain efficiency and accuracy in delivery.

Other designations mentioned, like "Return to sender," specifically deal with the process of returning mail to the original sender when it cannot be delivered. "Delayed mail" refers to mail that is still on its way to the recipient but has been delayed, rather than being undeliverable. "Untraceable mail" typically pertains to mail that cannot be tracked, rather than its delivery status. Thus, "Undeliverable mail" is the most accurate choice for describing mail that cannot be delivered.