Mailbox
Postbox
noun
郵便 is the "mail." A box that's used with said mail is a mailbox.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
どうやって私の小さな郵便箱にこの本が入ったんだろう。
How could this book fit in my small mailbox?
僕の家の前の表札の横には、郵便箱が一つ取り付けてあった。
There was one mailbox mounted next to the nameplate in front of my house.
この郵便箱、カメラがついてるよ！
This mailbox has a camera in it!
コウイチあてのラブレターがあまりにもたくさん届くので、巨大なタンカーを郵便箱の代わりに使っています。
Since so many love letters are delivered to Koichi, we use a gigantic tanker as a mailbox instead.