basically you can use any email address you like, even non-existing addresses, because this is just what is shown to the recipient, the background email address is always an address from the server (hoster) you send the email from.
$text = "<html><head><title>your title</title></head>
<body>
<!-- html here -->
</body>
</html>";
$recipient = "$email"; //Mailadresse
$sender = "anything@youlike.com";
$subject = "Anything you like";
$answer = "anything@youlike.com";
$hea = "MIME-Version: 1.0\r\n";
$hea .= "Content-type: text/html; charset=iso-8859-1\r\n";
$hea .= "From: $sender\r\n";
$hea .= "Reply-To: $answer\r\n";
mail($recipient,$subject,$text,$hea);