the mailto sends the email of the cron job to whatever email you want it to...   the -q in the cron command means quiet so there is no output.  But you can email yourself the cron if you want to be sure it ran.  As far as i know you dont have to have it... but i could be wrong. 

As root, I ran "crontab -e" and wrote

*****php72 /var/www/html/ow_cron/run.php

To be honest, I see that sometimes, the email notifications are sent and sometimes, aren't.

The /var/log/cron shows no malicious thing. For example, in a group, if user A sends a post, email notification is sent to B. However, not vice versa. The notification setting for both are the same. They are set as Immediate notification.

However, email notification for user login is sent for both users A and B.
Any more idea for debugging?