php contact form does not send emails through server

Question

The code below with the contact us page doesn't deliver the email through the server and shows the error message every time the user tries to send them.

Sorry " + name + ", it seems that our mail server is not responding. Please try again later!

contact.php:

<?php
if(empty($_POST['name']) || empty($_POST['subject']) || empty($_POST['message']) || !filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) {
  http_response_code(500);
  exit();
}

$name = strip_tags(htmlspecialchars($_POST['name']));
$email = strip_tags(htmlspecialchars($_POST['email']));
$m_subject = strip_tags(htmlspecialchars($_POST['subject']));
$message = strip_tags(htmlspecialchars($_POST['message']));

$to = "info@example.com";
$subject = "$m_subject:  $name";
$body = "You have received a new message from your website contact form.\n\n"."Here are the details:\n\nName: $name\n\n\nEmail: $email\n\nSubject: $m_subject\n\nMessage: $message";
$header = "From: $email";
$header .= "Reply-To: $email";  

if(!mail($to, $subject, $body, $header))
  http_response_code(500);
?>

contact.js:

$(function () {

    $("#contactForm input, #contactForm textarea").jqBootstrapValidation({
        preventSubmit: true,
        submitError: function ($form, event, errors) {
        },
        submitSuccess: function ($form, event) {
            event.preventDefault();
            var name = $("input#name").val();
            var email = $("input#email").val();
            var subject = $("input#subject").val();
            var message = $("textarea#message").val();

            $this = $("#sendMessageButton");
            $this.prop("disabled", true);

            $.ajax({
                url: "contact.php",
                type: "POST",
                data: {
                    name: name,
                    email: email,
                    subject: subject,
                    message: message
                },
                cache: false,
                success: function () {
                    $('#success').html("<div class='alert alert-success'>");
                    $('#success > .alert-success').html("<button type='button' class='close' data-dismiss='alert' aria-hidden='true'>&times;")
                            .append("</button>");
                    $('#success > .alert-success')
                            .append("<strong>Your message has been sent. </strong>");
                    $('#success > .alert-success')
                            .append('</div>');
                    $('#contactForm').trigger("reset");
                },
                error: function () {
                    $('#success').html("<div class='alert alert-danger'>");
                    $('#success > .alert-danger').html("<button type='button' class='close' data-dismiss='alert' aria-hidden='true'>&times;")
                            .append("</button>");
                    $('#success > .alert-danger').append($("<strong>").text("Sorry " + name + ", it seems that our mail server is not responding. Please try again later!"));
                    $('#success > .alert-danger').append('</div>');
                    $('#contactForm').trigger("reset");
                },
                complete: function () {
                    setTimeout(function () {
                        $this.prop("disabled", false);
                    }, 1000);
                }
            });
        },
        filter: function () {
            return $(this).is(":visible");
        },
    });

    $("a[data-toggle=\"tab\"]").click(function (e) {
        e.preventDefault();
        $(this).tab("show");
    });
});

$('#name').focus(function () {
    $('#success').html('');
});

Any help and explanation for why this error is showing?