By definition, spamming is illegal under the Controlling the Assault of Non-Solicited Pornography and Marketing Act of 2003 (the "CAN-SPAM Act"). Spamming is the transmission of any unsolicited "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)." (15 U.S.C. § 7702(2)(A).) The obligations of the Act apply to both the sender of the message and the person whose product, service, or web site is promoted by the message, both of whom are "senders" for purposes of the Act.
If there is a pre-existing business relationship between the sender and the e-mail recipient, and the recipient has not opted out of receiving e-mail communications from the sender, e-mail communications from the sender to that e-mail recipient would not constitute illegal spam, but those messages must include notification to recipients of their ability to decline receiving future email messages from, and a reply e-mail address or other mechanism that recipients can use to decline receiving future email messages from, you or the advertiser.
Violations can result in enforcement action against you by the FTC or by ISPs. Remedies include injunctive relief, disgorgement of profits, and actual damages or statutory damages or fines of $250 per violation, whichever is greater, with each unlawful message to each recipient being a separate violation. Statutory damages can go as high as $2 million. Certain fraudulent activities and repeat offenses include the possibility of imprisonment for three to five years.
Disclaimer: Please note that this answer does not constitute legal advice and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts, documents, and/or other materials involved. This answer does not create an attorney-client relationship.