Mailboxes which do have locking mechanisms must be able to have mail deposited inside them without the mail carrier requiring a key or a code. Locking mailboxes must also be able to pass a 3-minute test, whereby commonly available tools such as a pry bar would be unable to access the mail in the box. Mailboxes in the US need to meet certain requirements.Īccording to USPS-STF-7C directive outlined in 2015, any curbside mailbox, whether rural or in a city, must be able to accommodate a test parcel that measures 7 inches high, 13 inches wide, and 16 inches deep. Mailboxes should be made to fit with the most recent rules as set out by the USPS, the latest of which was the USPS-STF-7C directive outlined in 2015. There are specific regulations set by the USPS which have to be adhered to when it comes to mailboxes.