edit file /var/ossec/etc/ossec.conf.
change
<ossec_config>
<global>
<email_notification>yes</email_notification>
<email_to>[email protected]</email_to>
<smtp_server>mx.example.com..</smtp_server>
<email_from>[email protected]</email_from>
Set the Alter Level
<ossec_config>
<alerts>
<email_alert_level>10</email_alert_level>
Restart the Service
/var/ossec/bin/ossec-control restart