This page is identical to http://freebsd.qmailrocks.org/remove.htm, however sendmail is in

/usr/sbin/sendmail

and /etc/rc.conf should have 

sendmail_flags="-L sm-mta -C/etc/mail/localhost.cf -bd -q30m"

changed to

#sendmail_flags="-L sm-mta -C/etc/mail/localhost.cf -bd -q30m"
sendmail_flags=NO

rm /usr/sbin/sendmail

ln -s /var/qmail/bin/sendmail /usr/lib/sendmail
ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail

We should also remove the sendmail queue runner job from crontab. Usually the first of the jobs in the crontab:

*/30 * * * * /usr/sbin/sendmail -L sm-msp-queue -Ac -q