contact form not submitting

<?php

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

    $name = $_POST['name'];
    $email = $_POST['email'];
    $message = $_POST['message'];


    $to      = 'example@gmail.com';
    $subject = 'Site Contact Form';

    $headers = 'From: '. $email . "\r\n" .
    'Reply-To: '. $email . "\r\n" .
    'X-Mailer: PHP/' . phpversion();

    $status = mail($to, $subject, $message, $headers);

    if($status == TRUE){
        $res['sendstatus'] = 'done';

        //Edit your message here
        $res['message'] = 'Thanks for the message! I will be in touch soon.';
    }
    else{
        $res['message'] = 'Failed to send mail. Please email me at example@gmail.com instead';
    }


    echo json_encode($res);
}

?>