You have installed chkrootkit and it's now running with daily cron, but unless you are logging in daily to check the logs, you won't know of any potential problems. Here's a simple way to have the daily report emailed to you with only postfix installed, using the sendmail command.

Edit /etc/chkrootkit.conf and add the following

REPORT_EMAIL="[email protected]"

Edit /etc/cron.daily/chkrootkit and towards the bottom of the file, replace $CHKROOTKIT $RUN_DAILY_OPTS with

$CHKROOTKIT > $LOG_DIR/chkrootkit.log #Run chkrootkit and save the logfile
(
echo "Subject: [chkrootkit] $(hostname -f) - Daily report" #Create subject line using the hostname
echo "To: $REPORT_EMAIL" #Insert To: delivery email address using the variable defined in chkrootkit.conf
echo ""
cat $LOG_DIR/chkrootkit.log #Concat the logfile to the output
) | /usr/sbin/sendmail $REPORT_EMAIL #Send out the email!

It is probably a good idea to rotate the logs, so head over to /etc/logrotate.d/chkroot (new file) and enter

/var/log/chkrootkit/*.log {
  daily
  rotate 15
  dateext
  delaycompress
  missingok
  notifempty
}

Now wait for your daily scheduled cron to run, or run /etc/cron.daily/chkrootkit as root to test it out.