Mass emailing in the United States must comply with the CAN-SPAM Act. For effective email marketing services, every email must include an easy opt-out method for users to unsubscribe from receiving emails. This can be included anywhere in the email, but the link is usually at the bottom of the email.
Even though the CAN-SPAM Act does not have an opt-in requirement, meaning that you do not need to get the recipients' consent before sending them emails, we highly recommend that you do so. This will allow you to have a greater click-through rate and increase sales and revenue.
You must also provide access to the data that you collect on your subscribers, they must be able to exercise their rights to modify the information collected, cancellation or portability of their data.