Posts and other supports for curbside boxes are owned and controlled by customers who are responsible for seeing that posts are neat and of adequate strength and size. Customers are encouraged to avoid using massive mailbox supports that, when struck, could damage vehicles and cause injury. Heavy metal posts, concrete posts and miscellaneous farm equipment, such as milk cans filled with concrete, are examples of potentially dangerous supports. The ideal support is an assembly that bends or falls away when struck by a vehicle. Customers may attach the box to a fixed or movable arm.

The Federal Highway Administration (FHWA) has determined that wooden mailbox supports no larger than 4 inches by 4 inches, or a 2 inch diameter standard steel or aluminum pipe, buried no more than 24 inches, should safely break away if struck by a vehicle. The mailbox must also be securely attached to its post to prevent separation if struck.