According to the United States Postal Service, mail with insufficient postage is marked to show the difference in price and delivered to the recipient. The recipient must then pay the difference. Mail that has no postage is returned to the sender if a return address is listed.
If the sender tries to mail more than 10 pieces of mail with insufficient postage, the mail is not delivered to the recipient. Rather, the sender is notified of the discrepancy and must pay the difference before the mail will be delivered. Similarly, Express mail is not delivered until the sender provides sufficient postage.