Block Hotmail email addresses in user signup

I was recently asked to block users from signing up to a drupal site using a hotmail email address.

To achieve this I wrote a small module that will intercept the validate operation of the user module and add in an extra validation on the mail field of the user registration or edit form.

Create your dot info file: hotmail_check.info

; $Id$
name = Hotmail Check
description = "Removes the ability to login with a hotmail email address."
core = 6.x

Then create your dot module file: hotmail_check.module

<?php
// $Id$
/**
 * @file hotmail_check.module
 *
 * Hotmail Check Module
 *
 * This module removes the ability to login with a hotmail email address.
*/
function hotmail_check_user($op, &$edit, &$account, $category = NULL) {
  if ($op == 'validate') {
  // find @hotmail in the email address and return an error if it exists
    if (preg_match('/@hotmail/i', $edit['mail'])) {
      form_set_error('mail', t('E-mail addresses from Hotmail are not allowed.'));
    }
  }
}

And that is all that is required, so go ahead and install and enable the module, now users get an error if they try and use a hotmail.co.uk or hotmail.com email address to create a user account.