Comment 1 for bug 47597

As your system can display some Chinese the problem is more likely with the client application of the person sending you the email. That application is not marking the character encoding of the email, this is quite common with Hotmail and Foxmail. The workaround is to open up an email and force the email encoding: View > Character Encoding, and selected BIG5HKSCS for Traditional Chinese or GB2312 for SImplified Chinese, try the other options if these fail.

If you only communicate in one character set you can set the default in Edit > Preferences > Mail Preferences > Message Display > Default character encoding.

I would recommend telling the sender to simply upgrade their client application though as they will have this problem with every recipient not using the same program / webmail and character settings.