Breaking into a United States Federal Post Office, Post Office box, U.S. Postal Service delivery vehicle, street-corner U.S. mail deposit box, airplane or vehicle transporting U.S. mail, or personally robbing a U.S. postman or postwomen delivering mail is a Federal offense. Using the United States mail to defraud is also a Federal offense.
According to what I understand a lot of reservation-based Native Americans, because of the vast distances between farm and ranch homes (think of the expanse of the four-state Navajo Nation) reportedly often receive their mail at collective sites. I am sure that with a meaningful opportunity to vote by mail Native Americans will also hold caucuses similar to ours in Iowa, realizing that backing a certain individual may be in their best interest.