合格牛奶；保付支票；保证邮件。

Certified milk; certified check; certified mail.

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以数字方式签署邮件，以保证邮件发送的安全性。

Signs the message digitally to send the message securely.

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如果你在10天内没有得到答复，给代表打电话告诉他们你在等答复，寄另外一封有需求收据的保证邮件。

If you don't get a response within about 10 days, call the representative to let them know you're waiting for a response, and send another letter by certified mail with return receipt requested.

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