Custom UserController

CustomUserDomainPlugin.php

<?php
namespace Craft;

class CustomUserDomainPlugin extends BasePlugin
{
    /* --------------------------------------------------------------
    * PLUGIN INFO
    * ------------------------------------------------------------ */

    public function getName()
    {
        return Craft::t('Custom User Registration');
    }

    public function getVersion()
    {
        return '1.0';
    }

    public function getDeveloper()
    {
        return 'Josh Crawford';
    }

    public function getDeveloperUrl()
    {
        return 'http://sgroup.com.au';
    }

    public function init()
    {
        parent::init();
        
        craft()->on('users.onBeforeSaveUser', function(Event $event) {
            $user = $event->params['user'];
            
            $email = craft()->request->getPost('email', $user->email);
            $valid = true;
            
            if (!checkEmailIsAllowedDomain($email)) {
                $user->addError('email', Craft::t('Email supplied is not from allowed domain.'));
                $valid = false;
            }
            
            if (!$valid) {
                $event->performAction = false;
                
                craft()->urlManager->setRouteVariables(array(
			'account' => $user
		));
            }
        });
    }
}

register.html

{% macro errorList(errors) %}
    {% if errors %}
        {% for error in errors %}
            <label class="error">{{ error }}</label>
        {% endfor %}
    {% endif %}
{% endmacro %}

{% from _self import errorList %}

<form method="post" accept-charset="UTF-8">
    {{ getCsrfInput() }}
    <input type="hidden" name="action" value="users/saveUser">
    <input type="hidden" name="redirect" value="">
    
    <input placeholder="Email" type="email" name="email" required="required">
    
    {% if account is defined %}
        {{ errorList(account.getErrors('email')) }}
    {% endif %}
</form>

Create a directory called customuserdomain (all lowercase), and add the CustomUserDomainPlugin.php to it. Use the register form as normal.