No, there is no solution to this at the moment. There is a possible 
workaround, but that requires coding. See the mailing list archives 
for details.

yes, I knew that the problems is because of no specifing the encodeing 
method(like GB, Big5 ...) so that I see the ???? instead of correct 
character.

But I can't control the sender, for example, when a user in my server 
applying a phpBB forum a/c by using his a/c in my mail server and 
chose the language as chinese, the forum will send a confirmation 
email with such subject.

I just want to know, is there any solution to this problem.

Again, I can't control the email sender, all I want to do is handle 
the email with such subject.

You are probably seeing a broken message that doesn't properly specify 
the used charset in the subject header.

when I receive a email with encoded subject (Big5 Chinese), the 
subject in the horde email list become ???????.

I can't open that email too.