AJAX отправить с JQuery в PHP-файл не работает

Я хочу отправить информацию контактной формы на сервер и отправить контактную информацию по почте. Просто нормальная контактная форма.

Но я получаю ошибку по jquery.js в строке 9631, которая является этой частью:

//Do send the request
//This may raise an exception which is actually
//handled in jQuery.ajax (so no try/catch here)
xhr.send( ( options.hasContent && options.data ) || null );

Может кто-нибудь объяснить мне эту ошибку? Я не нахожу ошибку и не знаю, как ее исправить.

    <fieldset id="contactForm">
<p>Wünschen Sie eine Abholung von Schrott oder Edelmetallen<br>
geben Sie bitte <b>genaue Angaben über die Art und Menge</b> um weitere Rückfragen zu ersparen.
</p>
<hr>
<div class="contactField">
<div id="result"></div>
<label for="name"><span>Ihr Name</span>
<input class="frm-name email required" type="text" name="name" id="name">
</label>

<label for="email"><span>Ihre E-Mail</span>
<input class="frm-mail email required" type="email" name="email" id="email">
</label>

<label for="subject"><span>Betreff</span>
<input class="frm-mail subject required" name="subject" id="subject">
</label>

<label for="message"><span>Nachricht</span>
<textarea name="message" rows="10" cols="30" class="no-value frm-message required" id="message"></textarea>
</label>

<label>
<button class="submit_btn" id="submit_btn">Abschicken</button>
</label>
</div>
</fieldset>

JQuery

$("#submit_btn").click(function() {
//get input field values
var user_name       = $('input[name=name]').val();
var user_email      = $('input[name=email]').val();
var user_subject    = $('input[name=subject]').val();
var user_message    = $('textarea[name=message]').val();

//simple validation at client's end
//we simply change border color to red if empty field using .css()
var proceed = true;
if(user_name==""){
$('input[name=name]').css('border-color','red');
proceed = false;
}
if(user_email==""){
$('input[name=email]').css('border-color','red');
proceed = false;
}
if(user_subject==""){
$('input[name=subject]').css('border-color','red');
proceed = false;
}
if(user_message=="") {
$('textarea[name=message]').css('border-color','red');
proceed = false;
}

//everything looks good! proceed...
if(proceed)
{
//data to be sent to server
post_data = {'userName':user_name, 'userEmail':user_email, 'userSubject':user_subject, 'userMessage':user_message};

//Ajax post data to server
$.post('mail.php', post_data, function(response){

//load json data from server and output message
if(response.type == 'error')
{
output = '<div class="error">'+response.text+'</div>';
}else{

output = '<div class="success">'+response.text+'</div>';

//reset values in all input fields
$('#contact_form input').val('');
$('#contact_form textarea').val('');
}

$("#result").hide().html(output).slideDown();
}, 'json');

}
});

//reset previously set border colors and hide all message on .keyup()
$("#contact_form input, #contact_form textarea").keyup(function() {
$("#contact_form input, #contact_form textarea").css('border-color','');
$("#result").slideUp();
});

PHP

<?php
if($_POST)
{
$to_Email       = "mymail.com"; //Replace with recipient email address//check if its an ajax request, exit if not
if(!isset($_SERVER['HTTP_X_REQUESTED_WITH']) AND strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) != 'xmlhttprequest') {

//exit script outputting json data
$output = json_encode(
array(
'type'=>'error',
'text' => 'Request must come from Ajax'
));

die($output);
}

//check $_POST vars are set, exit if any missing
if(!isset($_POST["userName"]) || !isset($_POST["userEmail"]) || !isset($_POST["userMessage"])) || !isset($_POST["userSubject"]))
{
$output = json_encode(array('type'=>'error', 'text' => 'Input fields are empty!'));
die($output);
}

//Sanitize input data using PHP filter_var().
$user_Name        = filter_var($_POST["userName"], FILTER_SANITIZE_STRING);
$user_Subject   = filter_var($_POST["userSubject"], FILTER_SANITIZE_STRING);
$user_Email       = filter_var($_POST["userEmail"], FILTER_SANITIZE_EMAIL);
$user_Message     = filter_var($_POST["userMessage"], FILTER_SANITIZE_STRING);

//additional php validation
if(strlen($user_Name)<4) // If length is less than 4 it will throw an HTTP error.
{
$output = json_encode(array('type'=>'error', 'text' => 'Ihr Name ist zu kurz.'));
die($output);
}
if(!filter_var($user_Email, FILTER_VALIDATE_EMAIL)) //email validation
{
$output = json_encode(array('type'=>'error', 'text' => 'Das ist keine gültige E-Mail Adresse'));
die($output);
}
if(strlen($user_Subject)<2) //check emtpy message
{
$output = json_encode(array('type'=>'error', 'text' => 'Sie haben einen Betreff vergessen!'));
die($output);
}
if(strlen($user_Message)<5) //check emtpy message
{
$output = json_encode(array('type'=>'error', 'text' => 'Sie haben Ihre Nachricht vergessen!'));
die($output);
}

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

// send mail
$sentMail = @mail($to_Email, $user_Subject, $user_Message .'  -'.$user_Name, $headers);

if(!$sentMail)
{
$output = json_encode(array('type'=>'error', 'text' => 'Konnte Mail nicht verschicken. Wenden Sie sich an den Administrator.'));
die($output);
}else{
$output = json_encode(array('type'=>'message', 'text' => .$user_Name .' Vielen Dank für Ihre Nachricht.'));
die($output);
}
}
?>