What information must be on a piece of mail for it to be delivered?

For a piece of mail to be delivered, it is essential that it contains a complete address, which includes the recipient’s name and ZIP code. This is crucial because the mailing system relies heavily on this specific information to accurately route and deliver the mail. The recipient's name ensures that the mail is directed to the correct individual, while the ZIP code provides precise geographical information that helps postal workers determine the best path for delivery.

A return address, although helpful for undeliverable mail, is not mandatory for delivery. A tracking number is used for tracking purposes and is typically associated with packages rather than regular mail, while an order number is more relevant to specific transactions and does not influence the delivery process itself. Therefore, the most critical information required for successful delivery is a complete address that clearly identifies the recipient.