Woocommerce, the most popular ecommerce extension for wordpress, comes with a very efficient HTML template based automatic email system. The first thing to look for when customizing wordpress automatic emails is to go to the woocommerce/settings/emails area, where you can customize the subject of the emails for creating new account, new order, etc... If you want more customization , you have to go your theme and override the default woocommerce email templates.
After that you can test how mails react with the following extensions
- https://fr.wordpress.org/plugins/woo-email-control/ (be careful with that script though it has been reported as insecrure in the recent past - current version seems ok though - since version 1.02) - requires an order id, which is fine for most woocommerce installs, except for mine which is order less
- https://fr.wordpress.org/plugins/woocommerce-email-test/ only works for two templates
- Woo Preview Emails : fin too but also requires an email and should be integrated in woocommerce menu