I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Laws differ in each state: California
Under legislation approved in September 1998, unsolicited commercial e-mail messages must include opt-out instructions and contact information, and opt-out requests must be honored. Certain messages must contain a label ("ADV:" or "ADV:ADLT") at the beginning of the subject line. A provider may sue a sender of unsolicited commercial e-mail for violating the provider's policies if the sender has actual notice of such policies. The law applies to e-mail that is delivered to a California resident via a provider's facilities located in California.
Federal:
Unsolicited Electronic Mail Act of 2001 (H.R. 95)
H.R. 95 would require unsolicited commercial e-mail messages to be labelled and to include opt-out instructions, and would prohibit false routing information in such messages. It would prohibit the use of a provider's facilities to send unsolicited commercial e-mail in violation of the provider's policies, if the policies are clearly posted on a web site at the domain name included in the recipient's e-mail address or are made available by an FTC-approved standard method (presumably, via the provider's SMTP server).
H.R. 95, as introduced in January 2001, is identical to H.R. 3113 from the 106th Congress, in the form that bill was passed by the House of Representatives. The House Committee on Commerce published a report on H.R. 3113 in June 2000.
