挂号信

例句：

1. 1. We need to send a registered mail to inform them about the new policy.
   我们需要寄挂号邮件通知他们新的政策。
2. 2. The sender needs to register their letter before it can be sent.
   发件人需要在发送前登记他们的信件。
3. 3. It's important to register your mail so that it can be tracked if lost.
   注册你的邮件很重要，以防丢失时可以追踪。