Setting up NTP server is easy. But actually monitoring that server is a bit more difficult. A bare minimum should be getting an e-mail after reboot. However, even that simple step requires a bit of setup.

First you need to install sendmail, its configuration compiler, and a few SASL authentication methods:

yum install -y sendmail sendmail-cf cyrus-sasl-plain cyrus-sasl-md5

Next step is preparing authentication database (do substitute e-mail and password):

mkdir -p -m 700 /etc/mail/authinfo
echo 'AuthInfo: "U:root" "I:^^relay@gmail.com^^" "P:^^password^^"' > /etc/mail/authinfo/mail
makemap hash /etc/mail/authinfo/mail > /etc/mail/authinfo/mail

The last configuration step is adding the following lines into /etc/mail/sendmail.mc just ABOVE the first MAILER line:

…
define(`SMART_HOST',`[smtp.gmail.com]')dnl
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl
define(`confAUTH_OPTIONS', `A p')dnl
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confCACERT', `/etc/pki/tls/certs/ca-bundle.trust.crt')dnl
TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo/gmail.db')dnl
…

With configuration out of the way, we can proceed with “compiling” that new configuration and restarting the daemon:

make -C /etc/mail
systemctl start sendmail

Finally, we are ready to test e-mail via command line:

echo "Subject: Test via sendmail from `hostname`" | sendmail -v ^^youremail@example.com^^

Assuming everything works, the only remaining task is adding cron task (crontab -e):

@reboot  echo -e "Subject: `hostname` status\n\nHost rebooted at `date -R`." | /usr/sbin/sendmail -v ^^youremail@example.com^^

Now every reboot will result in a e-mail message.