Unfortunately, e-mails where the Chinese characters have all been converted to question marks are beyond fixing. Too much information is lost. Persons sending the e-mail should make sure to set the e-mail encoding to a Chinese encoding such as GB, Big5, or UTF-8.
For ideas on how to prevent e-mail from being corrupted in the first place, please visit the tutorial on e-mail at www.ChineseComputing.com. Also see Sending and Receiving E-Mail in Chinese under Windows: An Overview.
An off-line version of this service is now available as part of the DimSum Chinese Tools program.
The Hindi Unicode Repair Tool offers a similar web service for Hindi. For Cyrillic there is the Universal Cyrillic auto-decoder. The phenomenon of garbled text is often called mojibake from the Japanese version of the problem.
If you came to this page directly, you may also want to visit my other Chinese tools.