The only requirement of is that the sender must provide some mechanism whereby the receiver can opt out of future emails and that method cannot require the receiver to pay in order to opt out.

The CAN-SPAM Act

Explanation:

Email or electronic mail is an information system mechanism of communication, that involves the use of digital devices to initiate communication over the internet. The email uses several protocols like the POP3, IMAP etc.

The CAN-SPAM Act is proposed in the United States law, applicable to email investigations. It requires that a sender provides a mechanism whereby the receiver can opt out of future emails without a fee.