Prueba Form

HTML

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form action="" method="post" role="form" class="contactForm">
  <div class="form-group">
      <input type="email" class="form-control br-radius-zero" name="email" id="email" placeholder="Your Email" data-rule="email" data-msg="Please, enter a <b>email correct</b>." />
    <div class="validation"></div>
  </div>
  <div class="form-group">
  
    <input type="checkbox" name="LOPD" id="LOPD" data-rule="checked" data-msg="Please, accept" /><div class="validation"></div> I accept <a href="http://www.google.es/aviso-legal/" target="blank">conditions of use</a> and <a href="http://www.google.es/politica-de-privacidad/" target="blank">privacy policy</a>
  </div>
  <div class="form-action">
    <button type="submit" class="btn btn-form">Send Message</button>
  </div>
</form>

CSS

.validation {
    color: red;
    display:none;
    margin: 0 0 20px;
    font-weight:400;
    font-size:13px;
}

JavaScript

jQuery(document).ready(function($) {
  "use strict";

  //Contact
  $('form.contactForm').submit(function() {
    var f = $(this).find('.form-group'),
      ferror = false,
      emailExp = /^[^\s()<>@,;:\/]+@\w[\w\.-]+\.[a-z]{2,}$/i;

    f.children('input').each(function() { // run all inputs

      var i = $(this); // current input
      var rule = i.attr('data-rule');

      if (rule !== undefined) {
        var ierror = false; // error flag for current input
        var pos = rule.indexOf(':', 0);
        if (pos >= 0) {
          var exp = rule.substr(pos + 1, rule.length);
          rule = rule.substr(0, pos);
        } else {
          rule = rule.substr(pos + 1, rule.length);
        }

        switch (rule) {
          case 'email':
            if (!emailExp.test(i.val())) {
              ferror = ierror = true;
            }
            break;

          case 'checked':
            if (! i.is(':checked')) {
              ferror = ierror = true;
            }
            break;

        }
        i.next('.validation').html((ierror ? (i.attr('data-msg') !== undefined ? i.attr('data-msg') : 'wrong Input') : '')).show('blind');
      }
    });
    if (ferror) return false;
    else var str = $(this).serialize();
    $.ajax({
      type: "POST",
      url: "contactform/contactform.php",
      data: str,
      success: function(msg) {
        // alert(msg);
        if (msg == 'OK') {
          $("#sendmessage").addClass("show");
          $("#errormessage").removeClass("show");
          $('.contactForm').find("input").val("");
        } else {
          $("#sendmessage").removeClass("show");
          $("#errormessage").addClass("show");
          $('#errormessage').html(msg);
        }

      }
    });
    return false;
  });

});