nopCommerce's default email sender service supports SMTP method only, which deprives you of features provided by email delivery providers.
If you have a Mandrill account, this plugin allows you to use Mandrill API and its features for delivering system emails to your customers.
Guide for downloading the plugin :
first, add the plugin to your shopping cart and in order to download it, you should finish the checkout process.
after submitting an order, go to order details page or "packages" tab in your profile page. you'll be able to download the plugin via the provided link in the page.