The US Postal Service uses mules to deliver to Supai and Phantom Range, Range.

Animals have always been there to help us extend our messages across great distances. From messenger pigeons to the pony express; animals have delivered letters between lovers, parents and children, etc. 

In our modern time however, we’ve replaced many of these animals with mechanical vehicles, and most of those letters with e-mail. But, a few mules and delivery men still embark on six-to eight hour journeys through the Grand Canyon five days a week to deliver mail and other supplies to residents of Supai and Phantom Town, Arizona.

Both are relatively small towns located in mountainous regions. Supai for example, only has one small, air-conditioned lodge, one convenience store, and one café serving fast food. However, this has made both towns popular tourist destinations; allowing people to experience small town America with all its natural wonders and mule mail. 

(Source)