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?