I will recommend you to use EasySendy, Vero or AutoPilot. You should know anyway than sending emails is expensive and that should be considerate in your costs.
Sounds ambitious! What are the key challenges for you? Are you able to develop the product yourself, and just looking for ideas of how to grow it to millions of users? Or do you need to find people to build the product/app itself? There are lots of resources available for both.