While the Post Office permitted alternative designs for attached mailboxes and mail slots that met basic size and construction requirements, the same was not true for curbside mailboxes, which postal regulations required be in the form of the traditional dome-rectangular or 'tunnel-top' design first established in 1915.[from wikipedia]

Curbside mailboxes, known in Canada as rural mailboxes, must be weatherproof, have space for the name of the mailbox owner, and possess a signal device on the right-hand side (when facing the front of the mailbox) for pickup of outgoing mail.[from wikipedia]

In the US, wall-mounted or curbside mailboxes that are only designed for receiving incoming mail are known as "limited-service" mailboxes, while mailboxes equipped with a mechanism for notifying the postman to collect outgoing mail from the mailbox are known as "full service" mailboxes.[from wikipedia]

Rural and some suburban areas of North America may utilize curbside mailboxes, also known as rural mailboxes. These receptacles generally consist of a large metal box mounted on a support designed primarily to receive large quantities of incoming mail, often with an attached flag to signal the presence of outgoing mail to the mail carrier.[from wikipedia]