The issue and use of adhesive postage stamps continued during the 19th century primarily for first class mail. Each of these stamps generally bore the face or bust of an American president or another historically important statesman. However, once the Post Office realized during the 1890s that it could increase revenues by selling stamps as "collectibles," it began issuing commemorative stamps, first in connection with important national expositions, later for the anniversaries of significant American historical events. Continued technological innovation subsequently prompted the introduction of special stamps, such as those for use with airmail, zeppelin mail, registered mail, certified mail, and so on. Postage due stamps were issued for some time and were pasted by the post office to letters having insufficient postage with the postage due to be paid to the postal carrier at the receiving address.