A federal law that regulates commercial email.

The law can be defined in four parts:

1. It bans false or misleading header information
2. It prohibits deceptive subject lines.
3. It requires that your email give recipients an opt-out method.
4. It requires that commercial email be identified as an advertisement and include the sender's valid physical postal address.

http://www.ftc.gov/bcp/edu/pubs/business/ecommerce/bus61.shtm