Definition
Refers to replying to a letter in the next outgoing mail delivery after receiving it.
立即回信
I will reply by return upon receiving your message.
我將在收到您的消息後立即回覆。
Please reply by return after you receive this letter.
請在收到這封信後立即回覆。
They usually reply by return after receiving a letter.
他們通常在收到信後立即回覆。