16 CFR § 316.5 - Prohibition on charging a fee or imposing other requirements on recipients who wish to opt out.
Neither a sender nor any person acting on behalf of a sender may require that any recipient pay any fee, provide any information other than the recipient's electronic mail address and opt-out preferences, or take any other steps except sending a reply electronic mail message or visiting a single Internet Web page, in order to:
(a) Use a return electronic mail address or other Internet-based mechanism, required by 15 U.S.C. 7704(a)(3), to submit a request not to receive future commercial electronic mail messages from a sender; or
(b) Have such a request honored as required by 15 U.S.C. 7704(a)(3)(B) and (a)(4).