The [`email_exists()` function][1] contains the `email_exists` filter that you could use for this.  It receives the `$user_id` and the `$email`.  It will expect you to return either the user's ID (if the email exists), or a false boolean if it does not:

    add_filter( 'email_exists', 'my_custom_email_exists', 10, 2 );
    function my_custom_email_exists( $user_id, $email ) {
    
        /*
         * do your thing here...
         * just make sure to return the $user_id if the user exists
         * or a false boolean if they don't.
         */
        return $user_id;
    }


  [1]: https://developer.wordpress.org/reference/functions/email_exists/