Good morning on this sunny Friday.

Have you ever walked, letter in hand, to what you thought was a post office mailbox, only to find it sealed shut?

(We have.)

Did you notice the mailbox was green, not blue?

There are about 10,000 dark green boxes across the five boroughs that are marked U.S. MAIL, but they’re not for mailing letters.

We asked the United States Postal Service to learn why these green boxes exist and why they’re bolted shut.

They’re called “relay boxes” and they are used by letter carriers — like a cubbyhole, or a safe, for mail delivery.