A postmark is a postal marking made on a letter, package, postcard or the like indicating the (more or less precise) date and time that the item was delivered into the care of the postal service. Modern postmarks are often applied simultaneously with the cancellation or "killer" that marks the postage stamp(s) as having been used (though in some circumstances there may be a postmark without a killer, and sometimes the postmark and killer form a continuous design), and the two terms are often used interchangeably, if incorrectly. Postmarks may be applied by hand or by machines, using methods such as rollers or inkjets, while digital postmarks are a recent innovation. The local post Hawai'i Post had a rubber-stamp postmark parts of which were hand-painted. At Hideaway Island, Vanuatu, the Underwater Post Office has an embossed postmark.
The date of the postmark can be quite important. In the United States, the Internal Revenue Service will still consider income tax returns as filed on time though it receives them late if they are postmarked on time, and this date (with, perhaps, other proofs of mailing), may have significance as regards legal filings and proofs of service (though in this case the date may viewed as "on time" if the date of the postmark is no more than one day after the date service is supposed to have been made). Entries into sweepstakes and contests, and juried art exhibitions, may likewise have a "postmark deadline," and in at least one case it might have significance regarding the date of class withdrawal.
There are some examples of "faked covers" produced by philatelic forgers, most usually in order to increase their value, in which the postmark has been altered in some way; for example, by changing the date.
The "electronic postmark" was named by drawing a parallel with the regular postmark.
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.
Hawai'i post once had a surfboard mail postmark, for covers that traveled by surfboard.
While postmarks are applied almost universally by or under the authority of the official postal department, service, or authority [in the United States it is possible to receive a permit to apply your own postmark, called a Mailer's Permit Postmark]; it is at least theoretically possible that under certain conditions specified by the private express statutes in the United States, a privately carried letter may be cancelled with a private postmark. Unofficial entities that issue artistamps may use postmark-like markings as well.
Much of the published work on postmarks covers postmarks from before 1900. (This is perhaps because in the United States so-called fancy cancels were prevalent in this period, with the cancelling device often hand-cut from cork by the postmaster in elaborate shapes such as flags, stars or shapes that were seasonally-appropriate such as turkeys for Thanksgiving). Much work in studying postmarks is needed for 1900 and later.
In Great Britain the first postmark employed for the cancellation of the then new postage stamps was the Maltese Cross, so named because of its shape and appearance. This was used in conjunction with a date stamp which was applied, usually to the rear of the letter, which denoted the date of posting.
Fewer postmarks are used now than previously, with the advent of meter labels, which indicate the precise date and time of acceptance at the post office, some types of computer vended postage, and computerized postage people can print off their own PCs (called in the United States PC Postage, these services were offered by such companies as Stamps.com and Neopost, Inc.). These indicia do not need to be postmarked, though occasionally they are redundantly, and inadvertently (or for whatever other reason).
When the first universal postal system was started in the United Kingdom with its Penny Black, the postmark used red ink for contrast. This was not successful, and the stamp was changed to non-black colours so that the postmark could use black ink.
The majority of postmarks today are in black, with red (particularly in the United States with local post offices' handstamps) following, though sometimes they are in other colours. This is particularly true in the case of pictorial postmarks if the colour in question has some connection to the commemoration.
The Postmark Award is given to outstanding employees of Canada Post.
Flight cachets, more or less elaborate rubber-stamps on an envelope indicating on which flight (typically a first flight) a cover has traveled via air mail, are in addition to the postmark and are not postmarks either.
Source: "Collecting those strange Tongan stamps — on cover," in Scott Stamp Monthly (August 2002)