<P> Different types of postmarks include railway post offices ("RPOs") and maritime (on - board ship) postmarks . Postmarks on naval vessels during sensitive operations in wartime are sometimes "clean", showing less information than normally to prevent route of travel or other details from falling into enemy hands . Similar to this is the "censored postmark," overprinted with a black obliteration of the time and place of mailing for similar reasons . </P> <P> The Pony Express used a variety of different postmarks on the mail it carried across the Western United States . There are only 250 known examples of surviving Pony Express mail / postmarks in various collections today bearing one of more than a dozen different types of postmarks . </P> <P> Hawai'i post once had a surfboard mail postmark, for covers that traveled by surfboard . </P> <P> A colour postmark is on the United States Postal Service - issued collectible envelope commemorating the 2004 inauguration of George W. Bush . </P>

What does it mean to postmark a check