PHP和mail（）函数

Question

I am using a PHP script to send e-mails to a list of friends. This used to work successfully but now the only e-mails that are sent successfully are those inside my domain ("mycompany.com"). Any e-mails outside my domain fail to be sent. Is this a problem with permissions? Could I fix it by changing the headers I use when I call the mail() function? Here is the PHP script:

$name = "Joe Smith";
$email = "[e-mail]";
$to = "[e-mail], [e-mail], [e-mail], [e-mail]";
$subject = "Dinner Invitations";
$message = "Invitation to Dinner";
$headers  = "From: ".$name."<".$email.">
";
$headers .= "Reply-To: ".$email."
";
$headers .= "Return-Path: ".$email."
";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "
"; 
$headers .= 'Content-Transfer-Encoding: quoted-printable' . "
";
mail($to, $subject, $message, $headers);

Only [e-mail] was sent out by the server. [e-mail], [e-mail], and [e-mail] did not get sent.

Any suggestions would be deeply appreciated!