Post office postmarks are black ink lines printed by the U.S. Postal Service on the address side of each piece of stamped mail. They are used as a form of tracking, showing the date that the mail piece was received by the post office.
Postmarking a letter or package also cancels the postage that was originally affixed for mailing. This prevents the recipient from reusing the postage on another piece of mail. There are three methods for postmarking envelopes and packages, including automated canceling machines that are sensitive enough to detect hazardous materials, mechanized machines that date back to the early days of machine canceling, and hand-stamp devices that are used by postal employees in smaller post office settings.