letterset

例句：

1. 1.The letterset is organized by date and sender.
   信件集合按日期和发件人组织。
2. 2. The post office has a large collection of stamps.
   邮局有一大套邮票。
3. 3.They exchanged sets of letters to keep in touch.
   他们交换了信件集以保持联系。