The United States Postal Service (USPS) provides guidelines for the design, size and installation of mailboxes that receive deliveries from the USPS. Mailbox height requirements are the same for rural residents as for city or suburban dwellers.
TL;DR (Too Long; Didn't Read)
Mail-entry level, that is, where the mail delivery person sets the mail into the box, can be anywhere from 41 inches to 45 inches above ground.
Bottom-of-the-Box Height Requirement
The bottom of your rural mailbox can be no lower than 41 inches and no higher than 45 inches from ground level. These four inches of variance allow residents some range to affix their rural mailboxes while maintaining safety, mail integrity and aesthetics. A mailbox that's positioned too high or too low can slow down or even inhibit mail delivery by making it difficult for the driving or walking mail delivery person to place the mail safely and securely into the mailbox.
Installing a Rural Mailbox Post
Your rural mailbox's post should reach no more than 24 inches deep into the ground. That's because the most secure mailboxes are not only stable, but can bend or fall away if hit by a vehicle. A post that's buried too deep, or otherwise unyielding, is dangerous, according to the USPS. Avoid using posts made with farm equipment, such as concrete-filled cans, for your rural mailbox. Likewise, metal pipes and concrete posts have no flexibility upon impact. The Federal Highway Administration recommends 4-inch by 4-inch wooden posts or 2-inch metal posts made from aluminum or steel.
Other Rural Placement Rules
The face or opening of your mailbox should be 6 inches to 8 inches back from the curb, if there is one. If there is no raised curb in your rural neighborhood, contact your local postmaster for guidance on where precisely your mailbox must be placed. If your mailbox placement is such that it is located on a different street from your house or otherwise far away, your mailbox must be marked with your full street address, as opposed to just your house number.
Door Slot Height
Should your rural home come with a door slot for mail delivery rather than a mailbox, follow USPS' height guidelines for slots. The bottom of a mailbox slot must be at least 30 inches above the ground. The slot opening may not be wider than 1.5 inches and no longer than 7 inches.