This is a PHP problem not a Postfix problem.
Try this:
PHP Code:
$Mail_to = '[email protected]';
$subject= 'This is my subject...';
$Message= 'Hi, this is my mail message....';
$header = 'From: [email protected]' . "\r\n" .
'Reply-To: [email protected]' . "\r\n" .
'X-Mailer: PHP/' . phpversion();
mail($Mail_to, $subject, $Message, $header);