Hi Justus:

Would the following logic work for you:

  when receiving a new message on an issue, 
     check to see if the status of the issue is new (1)
     check to see if the submitter of the first message is the same as
         the new message
     if so keep the status as new
     if not change the status to chatting

This assumes that the "new" status is never reassigned. If it is
reassigned, then I guess scanning the history would be required.
So (1) would then read:

   check if the status of the issue is new and if
      the status has always been new (i.e. never changed)

so once the status of the ticket is changed, any email from anybody
would change it to chatting from new, done-cbb etc.

-- rouilj