It is the the law that has been in force since January, 1st 2004 and specifies the details of how e-mails should be sent.

If you are sending bulk e-mails via US based messaging service providers such as Gmail, Hotmail or Yahoo, you must follow the can-spam rules below:
You must include the “unsubscribe” link in your messages.
The sender’s e-mail address must be in operation and active.
Messages should not be sent to incorrect or inactive e-mail addresses.
Your email subject and content should be consistent.