Email Alert didn't work

I didn't have any mail alert when my device down,

I already set :
// Mailer backend Settings

$config['email_backend'] = 'sendmail'; // Mail backend. Allowed: "mail" (PHP's built-in), "sendmail", "smtp".
$config['email_from'] = '"Observium Network Monitor"<observium@domain>'; // Mail from. Default: "OBSERVIUM Network Monito$
$config['email_sendmail_path'] = '/usr/sbin/sendmail'; // The location of the sendmail program.
$config['email_smtp_host'] = 'localhost'; // Outgoing SMTP server name.
$config['email_smtp_port'] = 25; // The port to connect.
$config['email_smtp_timeout'] = 10; // SMTP connection timeout in seconds.
$config['email_smtp_secure'] = NULL; // Enable encryption. Use 'tls' or 'ssl'
$config['email_smtp_auth'] = FALSE; // Whether or not to use SMTP authentication.
$config['email_smtp_username'] = NULL; // SMTP username.
$config['email_smtp_password'] = NULL; // Password for SMTP authentication.

// Alerting Settings

$config['alerts']['email']['default'] = 'it.dctn@domain'; // Default alert recipient
$config['alerts']['email']['default_only'] = TRUE; // Only use default recipient
$config['alerts']['email']['enable'] = TRUE; // Enable email alerts
$config['alerts']['bgp']['whitelist'] = NULL; // Populate as an array() with ASNs to alert on.

$config['alerts']['port']['ifdown'] = TRUE; // Generate alerts for ports that go down
$config['alerts']['port']['ifdown_types'] = array('core', 'transit', 'peering'); // Generate alerts for ports that go down

$config['alerts']['interval'] = 86400; // How frequently to re-send a notification for a continuing alert condition // In seconds. Default is 1 day.

I already install mailutils, I try send mail via command mail and it was sending the email (OK),
but still I didn't get any email alert,

Please kindly help,
Thanks a Lot