Depending on your country and/or the clients you deserve, here are the main laws that are governing all email marketing practices.
USA: You need to comply with US CAN-SPAM.
European Union has written the General Data Protection Regulation (GDPR) that many member countries use either as a basis for their own laws or in lieu of a formal law.
Here are the links to their websites for complete information:
United States The CAN-SPAM Act: Requirements for Commercial Emailers
European Union General Data Protection Regulation (GDPR)
Australia Spam Act 2003