A sealed letter — a document placed in an envelope and sealed for delivery.
重要書類は封書で送付してください。
Please send important documents in a sealed envelope.
封書の封を開けずに提出するよう指示された。
We were instructed to submit the sealed letter without opening it.
履歴書は封書に入れて郵送した方が礼儀正しい。
It's more polite to send a resume enclosed in a sealed envelope.