The CAN-SPAM Act of 2003 established the first United States standards for senders of commercial email. It designates the Federal Trade Commission (FTC) as the enforcement body for this legislation.
CAN-SPAM has many stipulations for commercial email senders including requiring them to use appropriate subject lines, provide a physical address of the sender, offer recipients the ability to opt-out of future correspondence, and conspicuously label adult material.
CAN-SPAM is an acronym for Controlling the Assault of Non-Solicited Pornography and Marketing Act of 2003.
More information can be found here.