A postal marking is any kind of marginal note applied to a letter by a postal service. The most frequent types are postmarks and cancellations; almost every letter will have those, less common types include forward addresses, routing annotations, warnings, postage due notices and explanations, such as for smashed or delayed mail. A key part of postal history is the credit of postal markings, their purpose, and period of use.
Service marks give in sequence to the sender, recipient, or another post office. Advice marks notify about forwarding, miss ending, letters received in bad condition, letters received too late for delivery by a certain time, or the reason for a delay in mail delivery. Dead letter offices would use a variety of markings to keep track of their progress in finding the addressee, such as a notation that the letter had been advertise in the local newspaper. The tracking process for register mail may entail many marks and notations.
Shortly after the eruption of the American Civil War, the Northern authorities affirmed the existing postage stamps invalid and issued new types. Letters using the demonetized stamps received a marking "OLD STAMPS NOT RECOGNIZED", an accidentally humorous comment much prized by collectors today.