dead mail

American  

noun

1. undeliverable and unreturnable mail that is handled in the dead-letter office of the general post office.

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Example Sentences

“A lot of their dead mail, stuff that’s nonperishable, is given away to charity,” Tye said.

From Forbes

At my solicitation he readily moved for a return of all the letters, newspapers, magazines, pamphlets, &c., transmitted from the United States in 1846, and which have been refused on account of the rates of postage, and are consequently lying dead in the English post-office; also for a return of the amount of postage charged upon this dead mail matter.

From Project Gutenberg