codeigniter发送电子邮件但不是所有时间

this is driving me crazy.... I have just uploaded a site with this email configuration for receiving message from the contact form:

    $config = Array(
        'protocol' => 'smtp',
        'smtp_host' => 'mail.mysite.com',
        'smtp_port' => 587,
        'smtp_user' => 'noreply@mysite.com.ar', 
        'smtp_pass' => 'mypass', 
        'mailtype' => 'html',
        'charset' => 'iso-8859-1',
        'wordwrap' => TRUE
    ); 

    $this->email->initialize($config);
    $this->email->set_newline("
");

    $this->email->from($data['ct_email'], "contact form");
    $this->email->reply_to($data['ct_email']);
    $this->email->to("prueba@mysite.com");
    $this->email->bcc("noreply@mysite.com.ar"); //copy
    $this->email->subject($data['ct_subject']);
    $this->email->message($data['ct_message']);

    $email = false; 
    if($this->email->send()){     
        $email = true;   
    } 
    return $email;

As you can see I have a .com and a .com.ar, My server is configured with the .com.ar account but I created an alias with the .com

The problem is that messages arrive in 1 to 10 messages sended...so that means it only works sometimes.

How can I solve this? I have tried changing email addresses but it's all the same...

I keep trying sending messages and it only send sometimes. For the ones that are not sended this is the error displayed:

-0300 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail. Unable to send email using PHP SMTP. Your server might not be configured to send mail using this method.