A 'dead letter' refers to a piece of mail that cannot be delivered to its intended recipient nor can it be returned to the sender, making it effectively undeliverable.
First-Class Mail is a class of mail service that ensures rapid handling, delivery, free forwarding, and is not subject to opening for postal inspection. This service is generally used for sending letters, postcards, bills, and personal correspondence.
The main post office in a state responsible for receiving and distributing all mail within the state.
