Eso no es del postfix, es del php
Chekea
Código PHP:
<?php
$subject = "subject";
$message = "message"
//dont forget quotes around "name", else it won't be displayed correctly in utf-8
$from = "From: \"".$_POST['name']."\" <".$_POST['mail'].">";
mb_language('Neutral');
mb_internal_encoding("UTF-8");
mb_http_input("UTF-8");
mb_http_output("UTF-8");
mb_send_mail($to, $subject, $message, $from);
?>
Extraido de
http://es.php.net/manual/es/ref.mail.php