You need to add , 10, 3 after your closure to set the hook priority and (more importantly) the number of accepted arguments - otherwise $user_email and $errors will not be passed.

add_action( 'register_post', function ( $user_login, $user_email, $errors ) {
    $userIsValid = ValidateUser::make($errors);

    if(!$userIsValid) {
        $errors->add( 'bad_email_domain', '<strong>ERROR</strong>: errors' );
    }
}, 10, 3 );

You need to add , 10, 3 after your closure to set the hook priority and (more importantly) the number of accepted arguments - otherwise $user_email and $errors will not be passed.

add_action( 'register_post', function ( $user_login, $user_email, $errors ) {
    $userIsValid = ValidateUser::make($errors);

    if(!$userIsValid) {
        $errors->add( 'bad_email_domain', '<strong>ERROR</strong>: errors' );
    }
}, 10, 3 );

Update: The register_post hook only fires inside register_new_user(), which is only used for user registration via the WordPress login page.

To handle errors within the admin (i.e. Users > Add New) use the user_profile_update_errors hook instead.