quota setting

  Could you help me how to :  -set quota for each user  -block receiving mail if : used space + size of incoming mail >= 5MB   Until this time i've used this rules :   |if [ `du |tail -1|awk '{print $1}'` -ge `cat ../../mailquota-limit` ] ; then /var/qmail/bin/bouncesaying "User quota exceed

header problem

I'm new comer in  'qmail world'   I want to know : How to set Return-Path in header mail?