1-1.3 Postmarks
A postmark is an official Postal Service™ imprint applied in black ink on the address side of a stamped mailpiece. A postmark indicates the location and date the Postal Service accepted custody of a mailpiece, and it cancels affixed postage. Since 1979, the Postal Service’s Postal Operations Manual (POM) has provided standards for postmarks applied to single-piece First-Class Mail®. Letters and flats that need to be postmarked come from carrier pick-up, collection boxes, retail counters, or lobby drop boxes. Postmarks are not required for mailings bearing a permit, meter, or precanceled stamp for postage, nor to pieces with an indicia applied by various postage evidencing systems.
The postmarking process uses the following three basic methods of imprinting:
Automated: Advanced facer canceller systems used by processing distribution centers cancel letters quickly. These machines are equipped with biohazard detection systems so letters postmarked by automation benefit from added safety measures.
Mechanized: A variety of older devices apply postmarks to flat-size mailpieces and to philatelic pieces.
Manual: Hand-stamp devices are used by Postal Service employees for local cancellation or philatelic requests.
A “local” postmark shows the full name of the Post Office, a two-letter state abbreviation, ZIP Code™, and date of mailing. Because the Postal Service is sensitive to the importance some customers place upon these postmarks, each Post Office is required to make a local postmark available. Lobby drops should be designated for this purpose with clear signage signifying its use.
https://about.usps.com/handbooks/po408/ch1_003.htm