dead letter

English Thesaurus

1. mail that can neither be delivered nor returned (noun.communication)

hypernym	:	letter, missive,
definition	:	a written message addressed to a person or organization (noun.communication)

2. the state of something that has outlived its relevance (noun.state)

hypernym	:	state,
definition	:	the way something is with respect to its main attributes (noun.tops)