A partner's or affiliate's list, or maybe one you've purchased?
It's understandable that the instinct to give mass email to everyone out there is natural, especially if you're a home-based business or big organisation. https://Rubigordon.com/share/qr/?u=http%3a%2f%2flaunchhappy.co%2Fguides%2Fsquarespace-email-campaign-pricing-features-explained