How can I disable qmail from conducting reverse DNS lookups on SMTP connections?

This can be done by adding a "-H" flag to the tcpserver call within the qmail-smtpd supervise script. That file is located at /var/qmail/supervise/qmail-smtpd/run and you would do the following:

Find the line that starts with the tcpserver call:

/usr/local/bin/tcpserver -v -R -l "$LOCAL" -x /etc/tcp.smtp.cdb -c "$MAXSMTPD" \

and add the "-H" flag:

/usr/local/bin/tcpserver -v -R -H -l "$LOCAL" -x /etc/tcp.smtp.cdb -c "$MAXSMTPD" \