yum-cron is one of the jobs you don’t get emails for, right? You can try the following (won’t recommend it if you have automatic installation of updates):
grep ^email /etc/yum/yum-cron.conf
sed -i "/^random_sleep/ s/^/#/" /etc/yum/yum-cron.conf # no waiting; the same can be accomplished changing randomWait config prop
yum-cron # if there is any update shall email something
grep Yum $(find /var/lib/nethserver/vmail/root/Maildir/ -type f -mtime -1)
Undo changes in yum-cron.conf:
sed -i "/^#random_sleep/ s/^#//" /etc/yum/yum-cron.conf