According to 15 USCS § 7702 (2), [Title 15. Commerce and Trade; Chapter 103. Controlling the Assault of Non-Solicited Pornography and Marketing]
“(A) In general. The term "commercial electronic mail message" means any electronic mail message the primary purpose of which is the commercial advertisement or promotion of a commercial product or service (including content on an Internet website operated for a commercial purpose).
(B) Transactional or relationship messages. The term "commercial electronic mail message" does not include a transactional or relationship message.
(C) Regulations regarding primary purpose. Not later than 12 months after the date of the enactment of this Act [enacted Dec. 16, 2003], the Commission shall issue regulations pursuant to section 13 [15 USCS § 7711] defining the relevant criteria to facilitate the determination of the primary purpose of an electronic mail message.
(D) Reference to company or website. The inclusion of a reference to a commercial entity or a link to the website of a commercial entity in an electronic mail message does not, by itself, cause such message to be treated as a commercial electronic mail message for purposes of this Act if the contents or circumstances of the message indicate a primary purpose other than commercial advertisement or promotion of a commercial product or service.”