Hvorfor kan dette nyhedsbrev script ikke sende til mange brugere?

Nyhedsbrevet skal sendes til ca. 250 mail adresser men den kommer aldrig så langt ????

<?

    $emne = "Nyhedsbrev for uge " . date("W");
    $headers ="MIME-Version: 1.0\r\n";
    $headers .= "X-Mailer: Newsletter mailer\r\n";
    $headers .= "From: Me <my@email.dk>\r\n";
    $headers .= "Content-Type: text/html;\n  charset=iso-8859-1\r\n";
            if ($send == "yes"){
    $mailcount = 0;
    $lines = explode(",",$content);
        for ($key=1;$key<sizeof($lines);$key++){
            $mailslet = $lines[$key];
            include 'newsletter.php';
        mail($lines[$key],$emne,wordwrap(nl2br($design)), $headers);
        $mailcount = $mailcount + 1;
        }
    print "<CENTER><b>Mailen er sendt til: " . $mailcount . " Brugere!</b></CENTER>";
    print "<pre>$mailheaders\n$subject\n$message</pre>";
    }
    else { include 'newsletter.php'; echo "<center>" . $emne . "<br>" . $design . "</center>";
   
    ?>