to submit a valid opt-out request, a recipient cannot be required to pay a fee, provide information other than his or her email address and opt-out preferences, or take any steps other than sending a reply email message or visiting a single page on an Internet website

source: http://www.ftc.gov/os/2008/05/R411008frn.pdf

Note: if you are not telling me about a financial transaction we made, our email is not transactional and STILL must follow that ruling.