main.js

var clContactForm = function () {

    /* local validation */
    $('#contactForm').validate({

      /* submit via ajax */
      submitHandler: function (form) {

        var sLoader = $('.submit-loader');

        $.ajax({

          type: "POST",
          url: "inc/sendEmail.php",
          data: $(form).serialize(),
          beforeSend: function () {

            sLoader.slideDown("slow");

          },
          success: function (msg) {

            // Message was sent
            if (msg == 'OK') {
              sLoader.slideUp("slow");
              $('.message-warning').fadeOut();
              $('#contactForm').fadeOut();
              $('.message-success').fadeIn();
            }
            // There was an error
            else {
              sLoader.slideUp("slow");
              $('.message-warning').html(msg);
              $('.message-warning').slideDown("slow");
            }

          },
          error: function () {

            sLoader.slideUp("slow");
            $('.message-warning').html("Something went wrong. Please try again.");
            $('.message-warning').slideDown("slow");

          }

        });
      }

    });
  };

sendEmail.php

<?php

// Replace this with your own email address
$siteOwnersEmail = 'simone.robutti@gmail.com';


if($_POST) {

    $name = trim(stripslashes($_POST['contactName']));
    $email = trim(stripslashes($_POST['contactEmail']));
    $subject = trim(stripslashes($_POST['contactSubject']));
    $contact_message = trim(stripslashes($_POST['contactMessage']));

    // Check Name
    if (strlen($name) < 2) {
        $error['name'] = "Please enter your name.";
    }
    // Check Email
    if (!preg_match('/^[a-z0-9&\'\.\-_\+]+@[a-z0-9\-]+\.([a-z0-9\-]+\.)*+[a-z]{2}/is', $email)) {
        $error['email'] = "Please enter a valid email address.";
    }
    // Check Message
    if (strlen($contact_message) < 15) {
        $error['message'] = "Please enter your message. It should have at least 15 characters.";
    }
    // Subject
    if ($subject == '') { $subject = "Contact Form Submission"; }


    // Set Message
    $message .= "Email from: " . $name . "<br />";
    $message .= "Email address: " . $email . "<br />";
    $message .= "Message: <br />";
    $message .= $contact_message;
    $message .= "<br /> ----- <br /> This email was sent from your site's contact form. <br />";

    // Set From: header
    $from =  $name . " <" . $email . ">";

    // Email Headers
    $headers = "From: " . $from . "\r\n";
    $headers .= "Reply-To: ". $email . "\r\n";
    $headers .= "MIME-Version: 1.0\r\n";
    $headers .= "Content-Type: text/html; charset=ISO-8859-1\r\n";


    if (!$error) {

        ini_set("sendmail_from", $siteOwnersEmail); // for windows server
        $mail = mail($siteOwnersEmail, $subject, $message, $headers);

        if ($mail) { echo "OK"; }
        else { echo "Something went wrong. Please try again."; }
        
    } # end if - no validation error

    else {

        $response = (isset($error['name'])) ? $error['name'] . "<br /> \n" : null;
        $response .= (isset($error['email'])) ? $error['email'] . "<br /> \n" : null;
        $response .= (isset($error['message'])) ? $error['message'] . "<br />" : null;
        
        echo $response;

    } # end if - there was a validation error

}

?>