Updating user meta data from external link, user not logged in

I’m not entirely clear on what you’re trying to do, but it seems you want to pass the user’s email address via a query string and determine the user by that. You can get the user’s ID for writing user meta values from the email value. Use the function get_user_by() to retrieve the user object by the user’s email.

$email = ( isset( $_GET['login'] ) ) ? $_GET['login'] : false;

$user = get_user_by( 'email', $email );

if ( $user ) {
    update_user_meta( $user->ID, 'meta_key_to_save', $value );
}