Mailing lists

Hi,
I have a little problem with a mailing list: As I can see from the other  
messages posted in the list, the In-Reply-To:-Header is used to refer to  
other messages. So others can see which message replies to which.

My Problem is: Opera does not add this header (if I guessed correctly the  
header’s aim). So, the mail is added to the list as a new thread.

Is it possible to make that work correctly?

cheers,