Do you know USPS? You see, the United States Postal Service has specific requirements for all residential roadside mailboxes. The USPS Domestic Mail Manual surely describes the rules and regulations regarding housing roadside mailbox. Manual describes the standard and another letter to letterboxes container deposit or acceptance letter.
Do you know Residential Mail boxes? A residential mailbox is each side of the road or the walls of the container used for the purpose of receiving material in which the stamps are paid. You may find it as wall mount mailbox. Only official letter carriers can put the letters or packages into the mail container. No part of the mail container can be used to convey anything that does not contain a stamp. This includes items attached to the box, depending on the box or put in mailbox.
Slot door locked and non-waste that is used by apartment mailboxes are subject to the same rules as the roadside mailbox. Just keep in mind that a mail slot should be large enough to accommodate the type of letter received by the owner of the house. If the letter slot is not big enough, then the postman will stop delivering mail to houses.