A letter is postmarked when the United States Postal Service officially receives it and takes it into their custody. A postmark includes the full name of the post office in which it was processed and the state abbreviation, zip code and date of mailing.
Letters are postmarked after a carrier drops them off at the post office or after they are gathered from the lobby drop boxes. Letters deposited in collection boxes or left at the post office's retail counter are also postmarked once they are collected.
A postmark is not necessary for mail that is metered, has a permit or includes pre-canceled postage stamps. All other letters or flats, except for those that have an indicia that has been applied by a postage evidencing system, must be postmarked. The postmark cancels out the postage applied by the customer.
Postmarks may be applied manually or by an automated system. Requests can be made for hand postmarked pieces, and each local postmark is different. Automated systems can process large quantities of letters in a short time period and add an extra layer of security. As letters pass through the postmarking machine, they are scanned for bio-hazardous materials. An older process uses a mechanized system to apply postmarks.