dead letter

1. An unclaimed or undelivered letter that after a period of time is destroyed or returned to the sender by the postal service.
2. A law, directive, or factor still formally in effect but no longer valid or enforced.

Mailing piece that cannot be delivered as addressed or be returned to the sender. The U.S. Postal Service maintains a dead-letter branch that aids in locating letters reportedly not delivered. The final disposition of dead letters depends upon the class of mail and the nature of the material. The retention period varies from

5 days to 30 days in most cases. Mail that is insured, or that is cod is retained for six months. Unclaimed P.O. Box mail is kept as long as the box rental fee is paid. Registered mail is retained for 60 days. Foreign mail is turned over to the postal authorities of that country.

Letter that is undeliverable by the post office and cannot be returned to the sender.

1.  An unclaimed or undelivered letter that is eventually destroyed or returned to the sender. For example, She moved without leaving a forwarding address, so her mail ended up in the dead letter office. [c. 1700]
2.  A statute or directive that is still valid but in practice is not enforced. For example, The blue laws here are a dead letter; all the stores open on Sundays and holidays. [Second half of 1600s]

The noun has 2 meanings:

Meaning #1: the state of something that has outlived its relevance
  Synonym: non-issue

Meaning #2: a letter that can neither be delivered nor returned