Create custom validaton rule like:

<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        \Validator::extend('email_domain', function($attribute, $value, $parameters, $validator) {
        	$allowedEmailDomains = ['example.com', 'sub.example.com'];
        	return in_array( explode('@', $parameters[0])[1] , $allowedEmailDomains);
        });
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }
}

and then validate:

protected function validator(array $data) 
{ 
	return Validator::make($data, [ 
						   'name' => 'required|max:255',
						   'email' => 'required|email_domain:' . $data['email'] . '|max:255|unique:users',
						  'password' => 'required|min:6|confirmed', ]); 
}