Section 5(a) of the CAN-SPAM Act of 2003 sets forth the basic legal principles that differentiate legal and illegal commercial email. See 15 U.S.C. § 7704(a). According to these principles, the senders of commercial email will be engaging in legal activity, so long as:
1) The header of the commercial email (indicating the sending source, destination and routing information) doesn't contain materially false or materially misleading information;
2) The subject line doesn't contain deceptive information;
3) The email provides "clear and conspicuous" identification that it is an advertisement or solicitation;
4) The email includes some type of return email address, which can be used to indicate that the recipient no longer wishes to receive spam email from the sender (i.e. to "opt-out");
5) The email contains "clear and conspicuous" notice of the opportunity to opt-out of receiving future emails from the sender;
6) The email has not been sent after the sender received notice that the recipient no longer wishes to receive email from the sender (i.e. has "opted-out"); and
7) The email contains a valid, physical postal address for the sender.
According to the Senate Commerce Committee Report, violations of these principles constitute "unfair or deceptive acts or practices" that are subject to enforcement action by the Federal Trade Commission ("FTC"), and certain other Federal agencies. See S. Rep. No. 108-102 (2003), at 17.
For another summary of the core legal principles distinguishing legal from illegal commercial email, see the FTC's CAN-SPAM Act Compliance Guide for Businesses (September 2009).
It is important to understand that, while the usual meaning of "spam" is email sent in bulk to recipients who didn't ask for it, the CAN-SPAM Act (despite its name) only regulates commercial email, whether sent individually or in bulk. The CAN-SPAM Act does not address non-commercial, bulk email.