Simple email address validation - check syntax and MX record existing.

email_validation_simple.php

<?php
function _nxte_redeem_is_email_valid($email) {
  if (empty ($email)) {return false;}

  list($user_name, $mail_domain) = explode('@', $email);

  if (empty($user_name) || empty($mail_domain)) {return false;}

  $is_syntax_ok = (bool) filter_var($email, FILTER_VALIDATE_EMAIL);
  $is_MX_exists = checkdnsrr($mail_domain, 'MX');

  return ($is_syntax_ok && $is_MX_exists);
}