Hi Jerome,
if you want to debug this yourself, try to add some print
or logging statements to roundup/mailgw.py like
  print repr(original_msg)
to see what is going one.

Otherwise can you try to create something that is more reproducable?
Using some encoding for the Chinese characters 
and the email you are trying it with?
So it can be entered within python code in ascii so we call can be sure
that we are using excately the same bytes...