php simple check valid mail and mail server to prevent bounce

mail-check.php

<?php
//include 'functions.php';
$email = $_POST['mail'];
if(filter_var($email, FILTER_VALIDATE_EMAIL)) {
  $domain = array_pop(explode('@', $email));
  if(checkdnsrr($domain, 'mx')) {
    exit('mail sent');
    //mail($email, $subject, $message, $headers);
  } else {
    exit('mail server not found');
  }
} else {
  exit('mail doesn\'t valid');
}