WordPress at a glance
filter is not described

user_registration_email filter-hook . WP 2.1.0

Filters the email address of a user being registered.

Usage

add_filter( 'user_registration_email', 'filter_function_name_2463' );
function filter_function_name_2463( $user_email ){
	// filter...

	return $user_email;
}
$user_email(string)
The email address of the new user.

Where the hook is called

register_new_user()
wp-includes/user.php 2353
$user_email = apply_filters( 'user_registration_email', $user_email );

Where the hook is used (in WP core)

Does not used.