List quality determines whether your campaigns actually reach real inboxes or bounce, get flagged as spam, and hurt your sender reputation. A high-quality list is made of active, correctly formatted, and genuinely interested addresses rather than just a large volume of contacts. Before you send to any list, purchased or built, it’s worth checking how leads differ from subscribers so you know what you’re actually working with. See our guide on email leads vs email subscribers for the full picture. For sending benchmarks, check Mailchimp’s marketing data.