Error on contact form!

<?php

/* =====================================================
* change this to the email you want the form to send to
* ===================================================== */
$email_to = "[email protected]";
$email_subject = "Contact Form submitted";


if(isset($_POST['email']))
{


function return_error($error)
{
echo $error;
die();
}


// check for empty required fields
if (!isset($_POST['name']) ||
!isset($_POST['email']) ||
!isset($_POST['message']))
{
return_error('Please fill in all required fields.');
}


// form field values
$name = $_POST['name']; // required
$email = $_POST['email']; // required
$contact_number = $_POST['contact_number']; // not required
$message = $_POST['message']; // required


// form validation
$error_message = "";


// name
$name_exp = "/^[a-z0-9 .\-]+$/i";
if (!preg_match($name_exp,$name))
{
$this_error = 'Please enter a valid name.';
$error_message .= ($error_message == "") ? $this_error : "<br/>".$this_error;
}


$email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
if (!preg_match($email_exp,$email))
{
$this_error = 'Please enter a valid email address.';
$error_message .= ($error_message == "") ? $this_error : "<br/>".$this_error;
}


// if there are validation errors
if(strlen($error_message) > 0)
{
return_error($error_message);
}


// prepare email message
$email_message = "Form details below.\n\n";


function clean_string($string)
{
$bad = array("content-type", "bcc:", "to:", "cc:", "href");
return str_replace($bad, "", $string);
}


$email_message .= "Name: ".clean_string($name)."\n";
$email_message .= "Email: ".clean_string($email)."\n";
$email_message .= "Contact number: ".clean_string($contact_number)."\n";
$email_message .= "Message: ".clean_string($message)."\n";


// create email headers
$headers = 'From: '.$email."\r\n".
'Reply-To: '.$email."\r\n" .
'X-Mailer: PHP/' . phpversion();
if (@mail($email_to, $email_subject, $email_message, $headers))
{
echo 'Form submitted successfully.';
}
else
{
echo 'An error occured. Please try again later.';
die();
}
}
else
{
echo 'Please fill in all required fields.';
die();
}
?>

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if (@mail($email_to, $email_subject, $email_message, $headers))

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