$blacklist = array('gmail', 'gmx', 'hotmail');
$post = $_POST['check'];
$found = false;
foreach ($blacklist as $entry) {
  if (preg_match('/' . $entry . '/', $post)) {
    $found = true;
  }
}
if (empty($post)) {
  echo 'Type email (eg. dude@gmail.com)';
} else {
  if ($found) {
    echo $post . ' is blacklisted';
  } else {
    echo $post . ' is not blacklisted';
  }
}