VPS上的Codeigniter发送电子邮件失败，但未显示任何错误

Question

这是我使用的代码，

    $from = "thisistheemail@gmail.com";    //senders email address
        $subject = 'This is subject';  
        $message = 'This is message';

        //config email settings
        $config['protocol']  = 'smtp';
        $config['smtp_host'] = 'smtp.gmail.com';
        $config['smtp_port'] = '587';
        $config['smtp_user'] = $from;
        $config['smtp_pass'] = '******'; 
        $config['mailtype']  = 'html';
        $config['charset']   = 'iso-8859-1';
        $config['wordwrap']  = 'TRUE';
        $config['newline']   = "\r\n"; 
        $config['smtp_timeout'] = 30;
        $this->load->library('email', $config);
        $this->email->initialize($config);

        //send email
        $this->email->from('someone');
        $this->email->to($receiver);
        $this->email->subject($subject);
        $this->email->message($message);

该代码在Windows Xampp上运行良好并且成功发送了一封电子邮件，但在我的VPS上没有任何反应。请帮忙