When mail is undeliverable, it is marked with a yellow label. A code will be printed on the label to indicate the reasons why the mail couldn't be delivered. Most often, the reason is a change of address, missing or incorrect address information, a vacant location, or a change of address that has expired (COAs last for one year). Nixies are another resource mailers can use to keep their address list up-to-date. When mail is undeliverable, mailers must update or remove the address in question.