Both the CAN-SPAM Act and the regulations the Federal Trade Commission issued for enforcement of the CAN-SPAM Act contain the legal definition of a transactional message.  The definition from the CAN-SPAM Act may be found at 15 U.S.C. 7702(17) and the definition from the FTC regulations may be found at 16 C.F.R. 316.3(c).  The full definition is provided below.

(c) Transactional or relationship content of e-mail messages under the CAN-SPAM Act is content:

(1) To facilitate, complete, or confirm a commercial transaction that the recipient has previously agreed to enter into with the sender;

(2) To provide warranty information, product recall information, or safety or security information with respect to a commercial product or service used or purchased by the recipient;

(3) With respect to a subscription, membership, account, loan, or comparable ongoing commercial relationship involving the ongoing purchase or use by the recipient of products or services offered by the sender, to provide—

(i) Notification concerning a change in the terms or features;

(ii) Notification of a change in the recipient's standing or status; or

(iii) At regular periodic intervals, account balance information or other type of account statement;

(4) To provide information directly related to an employment relationship or related benefit plan in which the recipient is currently involved, participating, or enrolled; or

(5) To deliver goods or services, including product updates or upgrades, that the recipient is entitled to receive under the terms of a transaction that the recipient has previously agreed to enter into with the sender.