Hi all, here is little tuning for small qmail box with for valid user checking.

download patch to qmailsource directoryor anywhere you want.

wget http://code.dogmap.org/qmail/qmail-1.03-realrcptto-2006.12.10.patch
patch < qmail-1.03-realrcptto-2006.12.10.patch
make
qmailctl stop
make setup check

add the following at at /etc/tcp.smtp

:allow,QMAILRRTDENYALL="1"

save and exit.

qmailctl cdb
qmailctl start

WOW its works for me hope it will for you....