With MailWizz not only that you will be able to properly handle email marketing for your own purposes but you can also become an Email Service Provider for your customers, since MailWizz integrates easily with payment gateways like Paypal and it offers all the needed tools to create pricing plans, promotional codes and to manage orders and transactions created by your customers
Demo: https://codecanyon.net/item/mailwizz-email-marketing-application/6122150