You can only send emails to people who opt in to receive emails from your business. You can see those customers listed with an Accepts marketing indicator on their specific customer page or on your main Customers page. I’ve included some examples of what this may look like for you below. When you use Shopify Email, it will filter out any customers that do not accept email marketing for you.
I also wanted to share some resources that can help you out with your email marketing going forward. These blog and Help Center pages include some great tips on how to grow your subscriber list: