Send your roommate the notice via certified mail.
通过具有证明文件的电子邮件，通知室友。
You have to sign for it when you receive a certified mail.
您收到挂号信的时候要签收。
Certified mail protocol is fair exchange of a message m for a receipt.
认证邮件协议，指一个发送方要将消息M与收方收到的证据进行交换的协议。
Certified mail protocol is fair exchange of a message m for a receipt.
认证邮件协议，指一个发送方要将消息M与收方收到的证据进行交换的协议。
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