Dead letter, in a mail sorter’s vernacular

As a detective trying to solve the mystery of the crossword clue ‘Dead letter, in a mail sorter’s vernacular’, I first analysed the clue to understand the key phrases and meanings embedded within it. “Dead letter” meant that the letter in question was undeliverable for some reason, and “mail sorter’s vernacular” suggested that the answer would be a word or phrase commonly used in the postal service. With these clues in mind, I thought of a term I had heard before that fit these criteria – “NIXIE“. Upon further research, I discovered that this is indeed a common term used by mail sorters to refer to a letter that cannot be delivered and must be returned to the sender. This insight helped me fill in the answer with confidence and solve the mystery of the crossword clue.