The waterproof mailbox manufacturer tells you that the US Postal Service has established guidelines for mail recipients, including letter slot or mailbox size, location, and identification requirements. The waterproof mailbox manufacturer tells you that although the post office allows alternative designs for additional mailboxes and letter slots that meet the basic size and structure requirements, this is not the case for roadside mailboxes. The postal rules require the use of a traditional domed rectangle or "tunnel roof" form. The waterproof mailbox manufacturer tells you that the design was originally established in 1915. In 1978, seven years after the reorganized United States Postal Service was established, the postal authority finally approved the "modern" letter box specification as an alternative design.

The waterproof mailbox manufacturer tells you that currently, American roadside mail boxes are divided into (T) traditional, (C) contemporary or (L) locked. Traditional or modern non-locking roadside letter boxes are approved and available in three sizes: No. 1, No. 2, or No. 3, measured according to the smallest internal size.
The waterproof mailbox manufacturer tells you that the largest acceptable roadside mailbox is No. 3, with a length of 22.81 inches, a width of 11 inches, and a height of 15 inches (58 cm × 28 cm × 38 cm). The locked letter box is designed to provide security protection for the recipient’s incoming letters, with fewer restrictions on shape and size, but the design with a slot for sending and receiving letters must be at least 1.75 inches high and 10 inches wide.
The waterproof mailbox manufacturer tells you that according to the USPS regulations, a residential locked letter box cannot require the postal carrier to possess a key. Therefore, the USPS approved residential lock letter box does not have a safe outgoing letter. The waterproof mailbox manufacturer tells you that the installation requirements are different from the standard unlocked letter box: when using a locked letter box, the received letter slot must be 41 inches-45 inches above the curb surface, and the front of the letter box must be 6 inches away from the curb. Inches-8 inches. The USPS specification requirements for all letter boxes are the same, but the location of the "incoming mail area" varies depending on the locked letter box.