Glossary - CAN-SPAM

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/conline/pubs/buspubs/canspam.htm