When the United States Postal Service formally receives and assumes custody of a letter, it is postmarked. A postmark contains the complete name of the post office where the item was processed, the state abbreviation, the postal code, and the date of shipping.
After a letter has been delivered to the post office or retrieved from the lobby drop boxes, it is postmarked. Once collected, letters dropped in collection boxes or left at the post office’s retail counter are also postmarked.
Mail that is metered, has a permit, or has pre-canceled postage stamps does not require a postmark. All other letters and flats, excluding those with a postage-verification system-applied indication, must be postmarked. The postmark nullifies the customer-applied postage.
Postmarks can be added either manually or automatically. Requests for hand-affixed postmarks are possible, and each local postmark is unique. Automated systems can process a huge volume of mail in a short amount of time and add an additional degree of protection. As mail passes through the postmarking equipment, it is screened for biohazardous substances. Previously, postmarks were applied via a mechanical mechanism.