Dead letter

The term dead letter has several usages, each deriving from the notion of mail that is no longer deliverable.
Postal delivery.
In the field of postal delivery a dead letter is a letter, parcel, or other form of communication sent via the mail that for some reason is undeliverable or unclaimed. There are several scenarios that could lead to this situation. The letter may have been addressed to a nonexistent or undeliverable location ("e.g.", letters addressed to Santa Claus at the North Pole), a location which existed at the time of the sending of the letter but was destroyed or abandoned before it could be delivered ("e.g.", the World Trade Center or its ZIP code) with no forwarding address provided, or the address could have become unreadable. It is also possible that the letter was sent without postage or a return address. Depending upon the class of postage under which the letter is sent or the policy of the postal service in charge of the letter's delivery the letter will be returned to its originator (if possible) or destroyed. The United States Postal Service has a network of mail recovery centers, formerly known as "the dead letter office".
Law and policy.
A dead letter can also be anything that has outlived its relevance, such as a law which has not been revoked but is obsolete, inapplicable, or no longer enforced. This includes but is not limited to blue laws. Another example from an actual case is "Parker v. District of Columbia": "In short, we take the District’s position to be that the Second Amendment is a dead letter." 
In popular culture.
In Herman Melville's Bartleby the Scrivener, Bartleby was rumored to have worked in Washington DC's Dead Letter Office before arriving on Wall Street. Dead Letters were used as a metaphor for unfulfilled hopes.
The third episode in the first season of the television series Millennium was titled "Dead Letters".
The band Diary of Dreams has a song named "Dead Letter".
The band Angelspit has a song named "Dead Letter".
The band The Rasmus has an album named "Dead Letters".
The song "Stan" by popular rap artist Eminem deals with an obsessed fan, Stan, who regularly writes to Eminem. Already mentally disturbed, Stan's mental state and behaviour becomes even more deranged after he does not receive a reply for six months, and goes so far as to drive his car over a bridge with his pregnant girlfriend in the trunk. The music video to the song shows a number of mishaps at the post office which lead to most of Stan's messages becoming dead letters. Eminem finally responds to the one letter he does receive, but only after Stan's suicide.