Warning: Cannot modify header information - headers already sent by

    if ($_SERVER["REQUEST_METHOD"] == "POST") {

    $name = trim($_POST["name"]);
    $message = trim($_POST["message"]);
    $email = trim($_POST["email"]);
    $na_fill = trim($_POST["na_fill"]);

    ini_set('display_errors', 'On');
    error_reporting(E_ALL);

    if($name=="" || $message=="" || $email==""){
        $error_message = "Mandontory fields can't be empty.";
    }

    if(!isset($error_message)){
        foreach($_POST as $value){
            if( stripos($value,'Content-Type:') !== FALSE ){
                $error_message = "There was a problem with the information you entered.";
            }
        }
    }

    if($na_fill!="" && !isset($error_message)) {
        $error_message = "Your submission form has an error.";
    }

    require_once("class.phpmailer.php");

    $mail = new phpmailer();

    if(!isset($error_message) && !$mail -> ValidateAddress($email)){
        $error_message = "Your email is not valid.";
    }

    if(!isset($error_message)){
        $email_body = "";
        $email_body = $email_body. "Name: " . $name;
        $email_body = $email_body. "Email: ". $email;
        $email_body = $email_body. "Message: ". $message;

        $mail->SetFrom($email, $name);
        $address = "dlsm90@hotmail.com";
        $mail ->AddAddress($address, $name);
        $mail ->Subject = "Enquiry from phpmailer | ".$name;

        $mail ->MsgHTML($email_body);
        if($mail -> send()){
            header("Location: ?status=thanks");
            exit;
        }
        else {
            $error_message = "There was a problem sending the email: " . $mail->ErrorInfo;
        }
    }
} ?>

<?php
 if (isset($_GET["status"]) AND $_GET["status"] == "thanks") { ?>
        <p>Thanks for the email! I&rsquo;ll be in touch shortly!</p>
    <?php } else { ?>
    <?php
            if (!isset($error_message)) {
                echo '<p>I&rsquo;d love to hear from you! Complete the form to send me an email.</p>';
            } else {
                echo '<p class="message">' . $error_message . '</p>';
            }
        ?>
<form method="post">
    <input name="name" type="text"><br />
    <input name="email" type="email"><br />
    <textarea name="message" id="message"></textarea><br />
    <input name="na_fill" type="text" style="display:none;"><br />
    <input type="submit" value="Send">
</form>
<?php } ?>