The United States Postal Service maintains a list of the addresses to which it does not deliver mail on one or more days of the week. For example, the mailbox might reside on a rural delivery route that the United States Postal Service does not serve every weekday. Or, a business might inform the United States Postal Service that it does not operate a mailbox on one or more days of the week. The certified reference data files include the address list and the days of the week on which each address does not receive mail.