The United States Postal Service recommends that mailboxes be 41-45 inches off of the ground and 6-8 inches back from the curb with a minimum recommendation of 4 feet off the ground for those people living in the city. They also ask that everyone keeps a clear path to the mailbox so that in inclement weather the post carrier can reach the mailbox easily.
To keep the mailbox upright, it is recommended that the mailbox be 24 inches underground. Since the road and the curb conditions change from city to city and state to state, the USPS asks that people contact their local postmaster to ask for approval. This approval would be for the location of the mailbox before it is put up and for height and curb or house distance positioning.
For those individuals who are purchasing a new mailbox, the Postmaster General's seal of approval should be on it. Every new mailbox design is reviewed by the Postmaster General before it goes to market, so there is no reason not to choose an approved mailbox. For those individuals who insist on constructing their own mailbox, they should make sure that they run their plans by the local postmaster so that the mailbox meets the size, strength and quality standards found in the manufactured boxes.