Q: What do I need to configure when running yum-cron on CentOS 7?
 
A: 
  1. Make sure postfix is installed, autostarting and started.
  2. Install yum-cron.
    1. # yum install yum-cron
  3. Set autostart daemon.
    1. # systemctl enable yum-cron.service
  4. Start the daemon.
    1. # systemctl start yum-cron.service
    2. # systemctl status yum-cron.service
  5. Edit /etc/yum/yum-cron.conf.
  6. Check and change as appropriate the following lines:
    1. update_cmd = default
    2. update_messages = yes
    3. download_updates = yes
    4. apply_updates = no
    5. random_sleep = 0
    6. emit_via = email
    7. email_from = root@localhost>
    8. email_to = This email address is being protected from spambots. You need JavaScript enabled to view it.
    9. email_host = smtp.uu.se
  7. Restart the daemon for good measure.
    1. # systemctl stop yum-cron.service
    2. # systemctl start yum-cron.service
  8. Done.

Source: https://linuxaria.com/howto/enabling-automatic-updates-in-centos-7-and-rhel-7

Alse see Nightly linux updates doesn't workYum-cron settings for CentOS 6.