1) Kind of office 2) Law no longer enforced 
A dead-letter is a letter which lies for a certain period uncalled for at the post-office etc, or one which cannot be delivered from defect of address, and which is sent to the general post-office to be opened and returned to the sender.
[n] - the state of something that has outlived its relevance 2. [n] - a letter that can neither be delivered nor returned
dead mail noun mail that can neither be delivered nor returned
a law, ordinance, etc., that has lost its force but has not been formally repealed or abolished. · a letter that cannot reach the addressee or be returned to the sender, usually because of incorrect address, and that is sent to and handled in a special division or department of a general post office.