The USA has some of the least stringent email marketing regulations. The USA's applicable legilsation is the 2003 CAN-SPAM Act (Controlling the Assault of Non-Solicited Pornography and Marketing).
All emails must include a valid postal address. The subject line must also be reflective of the content. If you do not have prior consent to contact the recipient, it must be made clear that the message is commercial in nature.
CAN-SPAM, unlike most other email marketing legislations, works on an opt-out basis. There is no requirement for consent to contact subscribers based in the USA. You must make it clear how to opt-out and honor those requests promptly, within ten business days.
If you are sending emails without prior consent, check with your email provider first. The majority of email service providers (such as Mailchimp or EmailOctopus) require consent to contact your subscribers.
Legislation may vary on a state by state basis, with the applicable state dependent on where your subscribers are residents. One example is in California where the CCPA is enforced.
The CAN-SPAM Act is enforced primarily by the Federal Trade Commission (FTC), which can seek civil penalties of up to $16,000 per violation (with no maximum penalty). In certain circumstances the CAN-SPAM Act may also be enforced by other federal agencies including the Federal Communications Commission (FCC), state attorneys general, and Internet Service Providers (ISPs). There is no private right of action.