The USPS has a monopoly over the carriage of letters in and from the United States codified in a group of statutes known as PES. A provision of the PES allows the Postal Service to 'suspend [the PES restrictions] upon any mail route where the public interest requires the suspension.' In 1979, the Postal Service suspended restrictions for 'extremely urgent letters,' thereby allowing overnight delivery of letters by private courier services. P relied on that suspension to engage in a practice called 'international remailing.' This entails bypassing the Postal Service and using private courier systems to deposit with foreign postal systems letters destined for foreign addresses. The Postal Service issued a proposed modification and clarification of its regulation in order to make clear that the suspension for extremely urgent letters did not cover this practice. Because of the vigorous opposition to the proposed rule, the Postal Service agreed to reconsider its position and instituted a rulemaking 'to remove the cloud' over the validity of the international remailing services. Eventually, after more public meetings and comment periods, the Postal Service issued a proposal to suspend operation of the PES for international remailing. The Postal Service issued a final rule suspending the operation of the PES with respect to international remailing. Ps sued challenging the international remailing regulation pursuant to §702 (APA). The District Court granted summary judgment in favor of Ds. The appeals court held that Ps satisfied the zone-of-interests requirement for APA review under Clarke v. Securities Industry Assn. and that the Postal Service's regulation was arbitrary and capricious because it relied on too narrow an interpretation of 'the public interest.' The Supreme Court granted certiorari.