letter boxing

• 上下加框

letter boxing的用法和样例：

例句

1. He stamped the letter, sealed it off and then, put it into a letter box.
   他在信上贴好邮票，封好信口，然后把信投入信箱。
2. There are two collections a day from this letter box.
   这个邮箱每天开两次。