Dead letter.

Dead letter mail or undeliverable mail is mail that cannot be delivered to the addressee or returned to the sender. This is usually due to lack of compliance with postal regulations, an incomplete address and return address, or the inability to forward the mail when both correspondents move before the letter can be delivered. Largely based on the British model that emerged in the late eighteenth century, many countries developed similar systems for processing undeliverable mail.

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I think I think I think a little too often.