Check if Email already exist

I said that badly.
I would like to do a check when a user registers in the form. This is why I identified this action:
do_action( 'rwmb_profile_before_save_user', $object );
but I do not know how to use this action to make a check like this:

$email = 'myemail@example.com';
$exists = email_exists( $email );
if ( $exists ) {
    echo 'That E-mail is registered to user number ' . esc_html( $exists );
} else {
    echo 'That E-mail doesn\'t belong to any registered users on this site';
}