A mail is sent to root by default. I assume you set to send root mails to an external recipient in the root profile:

To send mails to other admins.
You may create a custom template but this disables mailing for yum-cron completely:
mkdir -p /etc/e-smith/templates-custom/etc/yum/yum-cron.conf/
cp /etc/e-smith/templates/etc/yum/yum-cron.conf/20emitters /etc/e-smith/templates-custom/etc/yum/yum-cron.conf/
Edit /etc/e-smith/templates-custom/etc/yum/yum-cron.conf/20emitters
and change emit_via = mail
to emit_via = None
Apply the changes:
signal-event nethserver-base-update
This is a “bad” custom template as it overrides an existing fragment and future updates to yum-cron may not apply correcty but I didn’t find another way.