To send something back; returning mail, packages, or documents to the sender (commonly used as 'to return/send back').
宛先不明のため荷物が返送されてきた。
The package was returned because the address was unknown.
注文と違う商品が届いたので、送料は店が負担して返送してくださいと言われた。
The wrong item arrived, so they told me to return it and that the store would cover the shipping.
書類に不備があったため、返送して修正を依頼した。
Because the documents were incomplete, they returned them and asked for corrections.