dead letter box

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dead letter box

or

dead letter drop

n
(Communications & Information) a place where messages and other material can be left and collected secretly without the sender and the recipient meeting
References in periodicals archive ?
They left notes in dead letter boxes or exchanged coded messages about geese flying south for the winter.
I've been training for the secret services for many years, since I was a child, in fact, making drops to dead letter boxes in the back garden, writing messages to my sister in lemon juice and concocting ciphers based on the page numbers in my favourite Enid Blyton books.