The situation is usually due to that the sender has used non-standard email system which does not specify the message encoding scheme. To properly view the email, you could try to select the suitable character set in the browser while reading the message (e.g. "BIG5" for Traditional Chinese, GB2312 for Simplified Chinese, or UTF-8 for Unicode).
For Internet Explorer (IE):
- Right click on the web page, then choose "Encoding" and select the code (e.g. BIG5).
- From the menu bar, click "View" -> "Character Encoding". Then select the code (e.g. BIG5).
Unfortunately, e-mails with Chinese characters having all been converted to question marks are beyond fixing. Too much information is lost. Senders should set the e-mail encoding to a Chinese encoding scheme such as GB, Big5, or UTF-8.
For more information, please refer to Wikipedia - 亂碼.