How to validate email address entered during checkout on Craft Commerce?

The craft commerce templates have too many checkout steps for us in practice. So I have combined them.

I have moved the email field into the Address page.

But I can submit this form without ever entering an email. There is no validation on it (at this stage anyway).

How can I validate the email at this point please?

To validate the address I use this, but when adding email to it there is an error because email is not part of the Address.

Event::on(
    Address::class,
    Model::EVENT_DEFINE_RULES,
    function(DefineRulesEvent $event) {
        $rules = $event->rules;
        $rules[] = [['firstName', 'lastName'], 'required'];
        $event->rules = $rules;
    }
);

For info, we are not allowing user registration, its guest accounts only. No need to check if a user email exists, or have a login form or a link to checkout as guest. We just need to check the email field has been filled in.