AJAX not submitting checkbox values to PHP

<div id="form-messages"></div>
<form class="green-txt" id="ajax-contact" method="post" action="mailer.php">
	  <div class="form-group">
	    <label for="name">Name:</label>
	    <input type="text" class="form-control" id="name" name="name" placeholder="" required>
	  </div>
	  <div class="form-group">
	    <label for="email">Email:</label>
	    <input type="email" class="form-control email-in" id="email" name="email" placeholder="" required>
	  </div>
	  <div class="form-group form-inline">
	    <label for="subscribe">Subscribe: </label>
	    <input class="sub-btn" type="checkbox" id="subscribe" name="subscribe">
	  </div>
	  <div class="row">
	  	<div class="col-xs-6 col-xs-offset-3 col-sm-4 col-sm-offset-8 col-md-4 col-md-offset-8">
	  	<button type="submit" class="btn-success btn btn-submit">Send</button>
	  	</div>
	  </div>
</form>

$(function() {

// Get the form.
var form = $('#ajax-contact');

// Get the messages div.
var formMessages = $('#form-messages');

// Set up an event listener for the contact form.
$(form).submit(function(e) {
	// Stop the browser from submitting the form.
	e.preventDefault();

	// Serialize the form data.
	var dataString = $(form).serialize();

	// Submit the form using AJAX.
	$.ajax({
		type: 'POST',
		url: $(form).attr('action'),
		data: dataString
	})
	.done(function(response) {
		// Make sure that the formMessages div has the 'success' class.
		$(formMessages).removeClass('error');
		$(formMessages).addClass('success');

		// Set the message text.
		$(formMessages).text(response);

		// Clear the form.
		$('#name').val('');
		$('#email').val('');

	})
	.fail(function(data) {
		// Make sure that the formMessages div has the 'error' class.
		$(formMessages).removeClass('success');
		$(formMessages).addClass('error');

		// Set the message text.
		if (data.responseText !== '') {
			$(formMessages).text(data.responseText);
		} else {
			$(formMessages).text('Oops! An error occured and your message could not be sent.');
		}
	});

});

});

// Only process POST reqeusts.
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Get the form fields and remove whitespace.
    $name = strip_tags(trim($_POST["name"]));
			$name = str_replace(array("\r","\n"),array(" "," "),$name);
    $email = filter_var(trim($_POST["email"]), FILTER_SANITIZE_EMAIL);
    $subscribe = trim($_POST["subscribe"]);


    // Check that data was sent to the mailer.
    if ( empty($name) OR !filter_var($email, FILTER_VALIDATE_EMAIL)) {
        // Set a 400 (bad request) response code and exit.
        http_response_code(400);
        echo "Oops! There was a problem with your submission. Please complete the form and try again.";
        exit;
    }

    // Set the recipient email address.
    // FIXME: Update this to your desired email address.
    $recipient = "";

    // Set the email subject.
    $subject = "UP News New Contact - $name";

    // Build the email content.
    $email_content = "Name: $name\n";
    $email_content .= "Email: $email\n";
    $subscribe = "Subscribe: $subscribe\n";



    // Build the email headers.
    $email_headers = "From: $name <$email>";

    // Send the email.
    if (mail($recipient, $subject, $email_content, $email_headers)) {
        // Set a 200 (okay) response code.
        http_response_code(200);
        echo "Thank You!";
    } else {
        // Set a 500 (internal server error) response code.
        http_response_code(500);
        echo "Oops! Something went wrong and we couldn't send your message.";
    }

} else {
    // Not a POST request, set a 403 (forbidden) response code.
    http_response_code(403);
    echo "There was a problem with your submission, please try again.";
}