What is a postmark on an envelope?

A postmark is a black ink stamp placed by the postal service on the address side of an envelope, usually directly over the postage stamp. Postmarks cancel the postage and indicate the date and location that the postal service received the envelope for processing.

As of 2014, the U.S. Postal Service requires postmarks only on mail collected from drop or collection boxes and mail picked up by carriers or mailed from post office lobbies. Postmarks are generally applied by machines, but they may be hand-stamped in local offices. Each post office in the United States is required to have a local postmark available upon request for customers who study or collect postmarks.