Depending on the nation or location, different laws apply to sending emails to receivers who did not subscribe. Spam, or unsolicited emails, are generally prohibited in most countries.
CAN-SPAM: United States
The CAN-SPAM Act is the federal law that governs commercial email in the United States. It establishes guidelines for commercial email, provides receivers the option to stop receiving emails from you, and outlines severe consequences for violators. Additionally, all commercial emails must offer an "unsubscribe" option that is active for at least 30 days after the email is delivered.
The Federal Trade Commission (FTC) in the United States provides information on the CAN-SPAM Act and how it applies to commercial email: CAN-SPAM Act: A Compliance Guide for Business
GDPR: European Union
The General Data Protection Regulation (GDPR) is in effect in the European Union. GDPR mandates that organizations get a customer's agreement before sending them direct marketing communications. Additionally, the receivers must have the choice to stop receiving further emails.
The Information Commissioner's Office (ICO) in the United Kingdom provides information on the General Data Protection Regulation (GDPR) and how it applies to electronic marketing: Guide to data protection and GDPR
CASL: Canada
The Canadian Anti-Spam Law (CASL) is in effect in Canada. Individuals must express their explicit or implied consent to receive commercial electronic messages under CASL, and they must be provided with a simple method to unsubscribe.
The Canadian Radio-television and Telecommunications Commission (CRTC) provides information on the Canadian Anti-Spam Law (CASL) and how it applies to commercial electronic messages: Spam and malware
Summary
- Always send emails to people who have signed in to receive them
- Always offer a simple unsubscribe option in every email you send.
If you apply by both of those rules on a regular basis then you shouldn't fall under any spam laws.
* It is important to note that inbox providers typically operate on a global basis. Therefore, any laws passed in a single major country or group of countries will have global enforcement at the inbox provider level.