>
> Also what are the best methods to prevent spam and make sure i am not
> acting as an open relay?

Prevent spam: no 100% solution AFAIK.
Use rbls ( xbl.spamhaus and njabl are fine )
and try spamassassin if you have the time to check its work ( check
false positives, train bayes database etc.. )

Open relay: check your /etc/tcp.smtp and virtualdomains and rcpthosts files
Make certain you don't have open proxies on you network ( apache for
example, etc)
Prevent qmail to bounce: take care you do not bounce in your
..qmail-default files ( vpopmail generates bounces by default )

Add this in your file:
---------
# rebuild smtp rules
[ -r /etc/tcp.smtp ] && tcprules $CDB
/var/service/qmail-smtpd/tcp.smtp.tmp < /etc/tcp.smtp

To make certain to have up-to-date rules before launching qmail-smtpd
----

>
> exec /usr/local/bin/softlimit -m 110000000 \
> /usr/local/bin/tcpserver -v -R -l "$LOCAL" -x /etc/tcp.smtp.cdb -c
> "$MAXSMTPD" \
> -u "$QMAILDUID" -g "$NOFILESGID" 0 smtp -r relays.ordb.org
> /var/qmail/bin/qmail-smtpd 2>&1

the -r options applies to rblsmtpd

Change your command to:
COMMAND="/usr/local/bin/recordio rblsmtpd -b -r combined.njabl.org -r
xbl.spamhaus.org qmail-smtpd"

recordio is useful to have a logging of rbl blocking
modify your
qmail-smtpd/log/run files with this:
exec setuidgid $LOGUSER multilog t $FILENO $FILESIZE '-* * > *' '-* *
< *' '+* * > 5*' '+* * > 4*' ./main
( last line )
So that you see the 553 errors generated by rblsmtpd