Easy Digital Downloads MailPoet:
The Easy Digital Downloads MailPoet extension integrates your eCommerce store with MailPoet. It helps by letting you automatically subscribe customers to specific lists based on what they purchased. It also allows customers to opt themselves into a list during checkout.
You can enable a “global” newsletter list, which all customers can have the option of signing up for during checkout. You can also select one or more lists that customers will be automatically subscribed to when purchasing specific products.