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sender; consignor; remitter
この荷物の送り主が不明だ。
The sender of this package is unknown.
配達員は送り主の住所を確認した。
The delivery driver confirmed the sender's address.
匿名で届いた贈り物に、送り主は誰だろうと考えた。
For the anonymously delivered gift, I wondered who the sender was.