"The CAN-SPAM Act establishes requirements for commercial messages, gives recipients the right to have you stop emailing them, and spells out tough penalties for violations." These are rules that all commercial emailers have to follow, which includes you if you are sending out bulk emails.
https://www.ftc.gov/tips-advice/business-center/guidance/can-spam-a...
https://www.ftc.gov/tips-advice/business-center/guidance/can-spam-a...