Don't make your potential client enter his email address to opt out, pass it in as a parameter. If you demand more than one click from him to opt out, then you

Don't make your potential client enter his email address to opt out, pass it in as a parameter. If you demand more than one click from him to opt out, then you are asking too much, it is after all your rudeness that has interrupted his day and is causing him to take action in the first place. In broader terms, there are companies that will handle this for you for a small fee. They also have fancy tricks to get through the bulk mail filters. If you send enough messages to get tagged as a bulk mailer you will probably have to change your domain name to get through in the future.