Проблемы с вложением PHP mail ()

Кто-нибудь может помочь мне выяснить, почему это продолжает возвращать false?

$to = "myemail@mydomain.com";
$from = "Website <website@mydomain.com>";
$subject = "Test Attachment Email";

$separator = md5(time());

// carriage return type (we use a PHP end of line constant)
$eol = PHP_EOL;

// attachment name
$filename = "document.pdf";

//$pdfdoc is PDF generated by FPDF
$attachment = chunk_split(base64_encode($pdfdoc));

// main header
$headers  = "From: ".$from.$eol;
$headers .= "MIME-Version: 1.0".$eol; 
$headers .= "Content-Type: multipart/mixed; boundary=\"".$separator."\"".$eol.$eol; 
$headers .= "Content-Transfer-Encoding: 7bit".$eol;
$headers .= "This is a MIME encoded message.".$eol.$eol;

// message
$headers .= "--".$separator.$eol;
$headers .= "Content-Type: text/html; charset=\"iso-8859-1\"".$eol;
$headers .= "Content-Transfer-Encoding: 8bit".$eol.$eol;
$headers .= $message.$eol.$eol;

// attachment
$headers .= "--".$separator.$eol;
$headers .= "Content-Type: application/octet-stream; name=\"".$filename."\"".$eol; 
$headers .= "Content-Transfer-Encoding: base64".$eol;
$headers .= "Content-Disposition: attachment".$eol.$eol;
$headers .= $attachment.$eol.$eol;
$headers .= "--".$separator."--";

// send message
if (mail($to, $subject, "", $headers)) {
    echo "mail send ... OK";
} else {
    echo "mail send ... ERROR";
}