dead letter

from The American Heritage® Dictionary of the English Language, 5th Edition.

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• noun A law, rule, or precedent still formally in effect but no longer valid or enforced.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An item of mail that cannot be delivered to its intended recipient; after some time it is returned to the sender, or destroyed
• noun by extension A law that is no longer enforced

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun mail that can neither be delivered nor returned
• noun the state of something that has outlived its relevance