[Answered ]-Adding a custom validation to Django's auth_user in admin on list_editable

Maybe the problem is that you should inherit your custom MyUserAdmin from UserAdmin from django’s admin models…

Here you can find an example of how to do that: Customizing an Admin form in Django while also using autodiscover

When you add a change like this, you should restart your server (restart manage.py or apache or whatever you are using)