Chinese characters in subject becomes ? when send to internet

Hi,

We have a user which is sending a mail with chinese characters, conversion in the body is okay, but the some chinese characters in the subject line turns to ??? when received by the user in the internet. Locally no problem is being encountered.

Here is the chinese character that is being send.

無法處理您的訂單需求 ← original characters

無法處理??? ← chracters in the receiving user in the internet.

Do you know what is causing this problem? The setting in the configuration document is set properly.