Lo metto così che è meglio.. So che non è corretta la cosa ma così si vede meglio...

<?php
$my_email = "daniele@conceptfbo.it"; //fill in your own e-mail adress
?>


<html>


<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<title>Contact Form</title>
<link href="contact_form.css" rel="stylesheet" type="text/css" />
<script>
function goBack()
{
window.history.back()
}
</script>
</head>


<body>


<?php
if (isset($_REQUEST['email'])) {
//send email
$email = $_REQUEST['email'] ;
$subject = $_REQUEST['subject'] ;
$message = $_REQUEST['message'] ;


echo "<table class='table' width='50%'>
<tr class='table_header'>
<td>Contact Form</td>
</tr>
<tr class='row1'>
<td>";
if ($email == "") {
echo "ERROR: you have to fill in a E-mail adress<br>";
echo "<input type='button' value='Back' onclick='goBack()' />";
exit;
}
if ($subject == "") {
echo "ERROR: you have to fill in a subject<br>";
echo "<input type='button' value='Back' onclick='goBack()' />";
exit;
}
if ($message == "") {
echo "ERROR: you have to fill in a message<br>";
echo "<input type='button' value='Back' onclick='goBack()' />";
exit;
}


mail($my_email, $subject,
$message, "From:" . $email);
echo "Thank you for using this form.<br>Your message has been send.
</td>
</tr>
</table>";

} else {


echo "<form method='post' action='contact_form.php'>
<table class='table' width='40%'>
<tr class='table_header'>
<td colspan='2'>Contact Form</td>
</tr>
<tr class='row1'>
<td>Email:</td>
<td>
<input name='email' type='text' />
</td>
</tr>
<tr class='row1'>
<td>Subject:</td>
<td>
<input name='subject' type='text' />
</td>
</tr>
<tr class='row1'>
<td valign='top'>Message:</td>
<td>
<textarea name='message' rows='8' cols='40'></textarea>
</td>
</tr>
<tr class='row1'>
<td>&nbsp;</td>
<td>
<input type='submit' value='Send Message' />
</td>
</tr>
</table>
</form>";
}
?>


</body>
</html>