is the post office required to deliver mail
The mission of the Postal Service is to provide the American public with trusted universal postal service. While not explicitly defined, the Postal Service's universal service obligation (USO) is broadly outlined in statute and includes multiple dimensions: geographic scope, range of products, access to services and facilities, delivery frequency, affordable and uniform pricing, service quality, and security of the mail. While other carriers may claim to voluntarily provide delivery on a broad basis, the Postal Service is the only carrier with a legal obligation to provide all the various aspects of universal service.
United States Postal Service