add_action('phpmailer_init', function( PHPMailer $phpmailer ){
    $phpmailer->isSMTP();
    $phpmailer->Host = 'smtp.mandrillapp.com';
    $phpmailer->SMTPAuth = true;
    $phpmailer->Port = 587;
    $phpmailer->Username = 'USERNAME';
    $phpmailer->Password = 'APIKEY';
    $phpmailer->SMTPSecure = 'tls';
    $phpmailer->addCustomHeader('X-MC-Subaccount', 'SUBACCOUNT');
    $phpmailer->setFrom('default@email.com', 'Default Title');
    $phpmailer->SMTPOptions = array(
        'ssl' => array(
            'verify_peer' => false,
            'verify_peer_name' => false,
            'allow_self_signed' => true
        )
    );

});