If you use something like MailChimp (I think that's what many new businesses tend to use? I can't remember..) or one with pretty subscribe/unsubscribe page for every customer who uses the mailing list service, then the customer's customers will have the luck to enjoy opt-in/opt-out option. If your organization is still running some out-of-date, custom subscription software, then unsubscribe might be out of the question.