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.
You can send an unlimited number of email campaigns (newsletters) to an unlimited number of lists containing an unlimited numbers of subscribers.
If you handle newsletter for multiple websites, you can treat those as customers and have unlimited too!