Отправка почты не работает, в чем причина?

 $to = "areostar@gmal.com";
    $subject = "This is subject";
         
    $message = "<b>This is HTML message.</b>";
    $message .= "<h1>This is headline.</h1>";
         
    $header = "From:support@stoxmarket.com \r\n";
    $header .= "Cc:afgh@somedomain.com \r\n";
    $header .= "MIME-Version: 1.0\r\n";
    $header .= "Content-type: text/html\r\n";
         
    $retval = mail ($to,$subject,$message,$header);
         
    if( $retval == true ) {
        echo "Message sent successfully...";
    }
    else
    {
        echo "Message could not be sent...";
    }

require_once('class.phpmailer.php');
 
        $mail = new PHPMailer(); // defaults to using php "mail()"
 
        $mail->IsSendmail(); // telling the class to use SendMail transport
        //$body             = file_get_contents('contents.html');
        //$body             = eregi_replace("[\]",'',$body);
        //$mail->AddReplyTo("name@yourdomain.com","First Last");
        $body="Test Mail";
        $mail->SetFrom('support@stoxmarket.com', 'stoxmarket support');
        //$mail->AddReplyTo("name@yourdomain.com","First Last");
        $address = "areostar@gmail.com";
        $mail->AddAddress($address, "Anton anton");
        $mail->Subject    = "PHPMailer Test Subject via Sendmail, basic";
        $mail->AltBody    = "To view the message, please use an HTML compatible email viewer!";
 
        $mail->MsgHTML($body);
        //$mail->AddAttachment("images/phpmailer.gif");      // attachment
        //$mail->AddAttachment("images/phpmailer_mini.gif"); // attachment
 
        if(!$mail->Send()) {
            echo "Mailer Error: " . $mail->ErrorInfo;
        } 
        else {
            echo "Message sent!";
        }