The United States Postal Service sets standards for curbside mailboxes under USPS-STD-7C, and publishes separate installation guidance for placement, wall-mount mailboxes, and door slots. Requirements differ depending on which you have, and several rules commonly attributed to the USPS actually come from elsewhere.
Locking mailboxes effectively prevent theft of incoming mail. They must meet the same functional requirements as standard mailboxes. Every Mail Boss post mount mailbox model is approved under USPS-STD-7C and listed in each year's Mailbox Improvement Week Postal Bulletin, found under Epoch Design.
- Slots for incoming mail must be at least 1.75 inches high by 10 inches wide.
- Protective flaps on locking mailboxes must be oriented inward so that mail carriers can place mail into the slot without additional effort.
- Locking mailbox slots must be large enough to hold the resident's normal daily volume of mail.
- Slots must also be large enough to accommodate unfolded US Priority Envelopes.
- Locks cannot be added to standard traditional or contemporary mailbox designs. Locking security mailboxes are approved under a separate USPS standard.
- USPS carriers do not open locked mailboxes and do not accept mailbox keys.