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.
A cancellation affixed on stamps by the U.S. Postal Service to indicate the use of postage. The date and place of mailing are usually indicated by the postmark and can be offered as evidence in a legal dispute of when and where something was mailed.
Special Delivery was a service provided by the U.S. Postal Service for expedited handling and delivery of mail, offering preferential processing and delivering even on Sundays and holidays.
