信封面上想叫人直接交畀收信人，唔好打開要點寫？（英文）

Just include the phrase "Private and Confidential"

usually you can write it on the back of the envelope

or

in the beginning of the letter before the content.


it is not strange to write that in an informal letter...

信封面上想叫人直接交畀收信人，唔好打開要點寫？

Well, if you put the following on the the envelope:
Please hand it to the direct receiver and not to be opened.
送信人或你的朋友肯定不開心, 覺得自己的信用在你心中有問題.

你應在信封面打上: Private and confidential 便可以. 意思是一樣的.

比個例子你吧：

ABC Ltd.
Unit 2B, Tai Man Industrial Building,
888 Tai Man Street,
Chai Wan, HK.
Attn : Mr. XXX

( PRIVATE & CONFIDENTIAL)

只要別人見到PRIVATE & CONFIDENTIAL的字眼，就不會亂開，
一定會交給收件人自己開。

xxxx co., Ltd
address

Attn: Mr.xxxxx In personal
Private and confidential

Personal & Confidential-Should Be Opened By The Addressee Only
私人密件-只供收件人開啟

Please don't open this letter if you are not the receiver.

After the address, you must put

Attention: Name

Also mark as "Urgent! Private & Confidential"

simple to state on the letter front : PLS PASS THIS MAIL TO xxx