How can you legally email to a list of people who haven’t opted in? Are you talking about transactional emails? (Receipts, etc?) or is this code for “I’m a spam

How can you legally email to a list of people who haven’t opted in? Are you talking about transactional emails? (Receipts, etc?) or is this code for “I’m a spammer.” Anyway.. sendgrid, AWS SES are good places to start.