Postmark is an official mark stamped on letters, package, postcard and other mail, serving as a cancellation of the postage stamp and indicating the date and time the item was delivered into the care of the postal service. Modern postmarks are often applied simultaneously with the cancellation that marks the postage stamps as having been used.
Postmarked is the verb of the term postmark and it means to stamp with such a mark. This term is relevant when it comes to date for filing applications. For Example : If the application form says it has to be postmarked on or before January 1st, 2010 it means that it has to be mailed on or before January 1st, 2010 and the postmark will reflect that.