When I send an e-mail using the mail function with a line 
"Bcc" in the headers, the 5th parameter does not work any 
more. I did not saw this type of bug in the list of already 
reported bugs.

Example :
  ........
  $headers  = "From: ".$mailfrom."\n";
  $headers .= "X-Mailer: PHP/".phpversion()."\n";
  $headers .= "MIME-Version: 1.0\n";
  $headers .= "Content-Type: text/plain; 
charset=\"iso-8859-1\"\n";
  $headers .= "Content-Transfer-Encoding: 8bit\n";
  $headers .= "Bcc: ".$notforyoureyes;

  $param = "-f ".$returnpath;

  if (mail($mailto, $subject, $message, $headers, $param))
    {
       ........
    }