Shopify recently updated the “Customers” section with much more powerful filters. For instance, you can now exclude customers with specific tags (impossible previously). Shopify Email leverages those filters to send to a segment of customers. I’m hopeful they will add the ability to filter based on marketing interactions (ie has not clicked the last 3 emails).
That being said, the Marketing app REALLY needs a simple checkbox for “resend to non-openers after X days”.