The purpose of a mailbox flag, properly known as a carrier signal flag, is to signal to the postal carrier that there is mail in the box that the customer wants picked up and delivered to its destination. Not all mailboxes have carrier signal flags since customers can choose to have a mailbox that only allows for incoming mail. For customers who do choose to use a mailbox with a carrier signal flag, the United States Postal Service (USPS) has requirements on its location, color and functionality.
TL;DR (Too Long; Didn't Read)
Look for mailboxes marked "U.S. Mail Approved by the Postmaster General," meaning that they have been found by USPS to meet all of its requirements. USPS also has on its website a listing of retailers that sell USPS approved parcel mailboxes, which are large mailboxes that can hold packages.
USPS Requirements for Mailboxes
A wide variety of mailboxes is available on the market for people to choose from. They vary in size, color, material, design and cost. While people can purchase whatever mailbox they like, it must meet USPS requirements. For standard curbside mailboxes, USPS has requirements for carrier door operation, the incoming mail opening, the outgoing mail retrieval and flag operation. The purpose of these requirements is so that postal carriers can easily and quickly deposit incoming mail and get outgoing mail.
USPS categorizes mailboxes into different types. Full service mailboxes allow for both incoming and outgoing mail to be placed inside the mailbox. These types of mailboxes can either be locked or unlocked, but both locked and unlocked full service mailboxes must have a carrier signal flag.
Purpose of a Carrier Signal Flag
When the flag is raised, it means that there is outgoing mail inside. The postal carrier will stop even if she has no incoming mail to deposit because she knows there is outgoing mail to be picked up. Some mailboxes currently available on the market have a metal clip located inside the door in which the outgoing mail is placed. After the postal carrier retrieves the outgoing mail and deposits any incoming mail, she will push the flag down. Flags that have a self-lowering feature go down on their own.
In contrast, postal carriers will not stop at mailboxes without flags unless they have incoming mail to deliver. Mailboxes without flags are limited service mailboxes, meaning they are designed for only incoming mail to be placed inside.
Carrier Signal Flag Requirements
The USPS requires that carrier signal flags be mounted on the right side of the mailbox when facing the mailbox from the front. This means that when the postal carrier or homeowner is facing the mailbox's front to deposit or retrieve mail, the carrier signal flag is on his right side.
The flag can't be too heavy nor too flimsy: it can't require a lift of more than two pounds of force to move up and down but must still remain in its position until moved by the postal carrier or customer. Mailboxes with self-lowering flags, meaning flags that automatically go down when the carrier service door is opened, are allowed provided that the postal carrier doesn't have to do anything else to move the flag.
As for the material of the carrier signal flag, it cannot be made of wood. USPS states that plastic is the preferred material. Whatever it is made of, it must be able to function correctly and continuously, without any breakage, at room temperature. It must be durable enough to be opened and closed at least 7,500 times.
Signal Flag Color Requirements
The flag can be any color except for any shade of white, green, yellow, brown or blue. USPS' preferred color is fluorescent orange. Whatever color for the carrier signal flag a customer chooses, it must have a clear contrast with the mailbox's color. The mailbox itself can be any color.
Carrier Signal Flag Design
USPS has some preferred mailbox and carrier signal flag designs. These designs can be found in the U.S. Postal Service Standard Mailboxes, Curbside publication, which is posted on the USPS website. All of the designs show a flag of 4 square inches at a minimum, which allows it to be easily seen. The flag should not have any sharp edges. While these designs are preferred, any design that meets USPS requirements may be allowed.