回函 (huí hán) — a written reply, a reply letter

Definición

回函 is a formal, written 'reply letter' — more official in tone than 回复 (huífù), which covers any response (including by phone or email).

verb

a written replya reply letter

回函。
Qǐng jǐn kuài gěi wǒ yì fēng huí hán.
Please give me a reply letter as soon as possible.
回函。
Wǒ men shōu dào le kè hù de huí hán.
We received the customer's reply letter.
回函。
Jīn tiān yào xiě yì fēng zhèng shì de huí hán.
Today we need to write a formal reply letter.