Crossword Genius

Undeliverable piece of mail (4,6)

Ross

I believe the answer is:

dead letter

(Other definitions for dead letter that I've seen before include "no longer observed" , "Law no longer enforced" , "Itll not lead to prosecution" , "Ineffective rule" , "Unresponsive landlord" .)

I've seen this clue in the Sydney Morning Herald.
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