The service providers are responsible for sending the emails to the users. You can choose any of the service providers to enable them to send the campaigns and track the analytics at the same time.
NotifyVisitors eases up the whole process of creating, scheduling, and tracking these email campaigns by choosing the service providers from the dashboard. You can choose the service provider and save it as default from the dashboard itself. Before that, the service providers must be integrated with the NotifyVisitors.