A Dead letter is mail that can neither be delivered nor returned to sender—see dead letter mail
It can also refer to:
Dead letter law, a type of unenforced law Dead letter queue, a sink queue when a message fails
Film and television The Dead Letter, a 1915 American silent comedy film starring Oliver Hardy Dead Letters (film), a 2007 American film "Dead Letter" (Murder, She Wrote), a 1989 television episode "Dead Letters" (Millennium), a 1996 television episode
Music Dead Letters, an album by the Rasmus, 2003 "Dead Letter", a song by Angelspit from Krankhaus, 2006 "Dead Letters", a song by Katatonia from Dead End Kings, 2012