[otrs] Notify sender when empty subject received

Hi list, I'd like to notify and drop any new mail received from a client when there is no subject in his mail. What kind of configuration should I use ? I was thinking of a combination of : - a postmaster filter forcing a special queue 'rejected-queue' - an auto-response for this queue saying that there must be a subject on their mails. But I'm not sure it this will do the trick. Any clues ? Thanks, Ionel