You know a mailbox when you see one. (They’re those blue hunks of metal bolted to the sidewalk with the creaky flaps that go reeeeaaaaaallllk when you pull them open.) But what about the dark-green boxes that don’t have any slots to accept mail?

Called postal relay boxes, these work as storage containers for mail carriers as they make their rounds. Carriers can replenish their bags on the go, removing the need to constantly return to the distribution center (or carry everything at once). They are most prevalent in cities where USPS workers make deliveries on foot, and the boxes are either filled by the carriers themselves or postal workers in trucks who make larger delivery runs.

Ideally, these relay boxes are put at the most convenient possible locations along carriers’ routes. A 1992 study in the American Journal of Mathematical and Management Sciences titled “Locating Postal Relay Boxes Using a Set Covering Algorithm” [PDF] details the number-crunching that goes into this. Using data from Canadian mail routes, the researchers took into account things like maximum mailbag weight (35 pounds), average mail volume (depending on day), and the number of mail carriers who can use each relay box at once. The algorithm resulted in a lower number of needed relay boxes, which cut down on cost.

You may have noticed that a green relay box that was present on your corner, say, ten years ago may no longer be there. As the Internet further reduces the need for paper mail, carrier loads have been getting lighter, accounting for fewer relay boxes. When Gothamist asked a USPS representative about the disappearing boxes, they confirmed that they were being removed “if they were no longer needed.” The rep was also tight-lipped about the very nature of the relay boxes, telling Gothamist’s Jen Carlson, “[They] are for official postal use only. Any further information regarding them is proprietary."

This reticence was likely due to security concerns. According to the Postal Inspection Service’s law enforcement guide, “relay boxes can contain large quantities of mail in gray sacks that thieves cart off looking for checks and credit cards.” It goes without saying, but please leave the relay boxes alone; they're just trying to help.