a letter that cannot be delivered to the address written on it and cannot be returned to the person who sent it

the dead-letter office 死信处理办公室

a law or agreement that is no longer effective

The ceasefire treaty was a dead letter as soon as it was signed, as neither side ever had any intention of keeping to it. 由于双方缺乏遵守的诚意，停火协定一经签署就成了废纸一张。