Icinga2: Send E-Mail on Host/Service Down

I installed and configured Icinga2 with icingaweb2.

I added a test-host with a test-service: Let's call it "example.com".

This is the test-host:

/etc/icinga2/conf.d/hosts/example-com:

object Host "example-com" {
        //Importiert die generische Host-Vorlage aus der template.conf
        import "generic-host"
        //IPv4 Adresse
        address = "2xx.2xx.2xx.2xx"

        //Angabe des Betriebssystemtyps
        vars.os = "Linux OS"
        vars.sla = "24x7"
}

Okay, then we have the service-configuration:

/etc/icinga2/conf.d/hosts/example-com/http.conf

object Service "http" {
    import "generic-service"
    host_name = "example-com"
    check_command = "http"
    vars.sla = "24x7"
}

Of course I added a new user group to /etc/icinga2/conf.d/users.conf

/**
 * The example user 'icingaadmin' and the example
 * group 'icingaadmins'.
 */

object User "icingaadmin" {
  import "generic-user"

  display_name = "Icinga 2 Admin"
  groups = [ "icingaadmins", "linux-admins" ]

  email = "[email protected]"
}

object UserGroup "icingaadmins" {
  display_name = "Icinga 2 Admin Group"
}

object UserGroup "linux-admins" {
  display_name = "Linux-Administrators"
}

Now I think I have to add the contact_groups to the host. But if I do that the config-check fails :-(

Thanks for your help :-)