If you want to send unlimited or very high volume emails like 1 million emails per day, you can do this by SMTP email server.
By taking service of already establishing email server or by setting up your own email servers.
Most of the email service provider will provide your email server on monthly rent basis or per email or per subscriber basis through which you can use their service with pay per use basis, but usually this cost you much higher.
The other solution is my setup your own SMTP server, through this you can send as many emails as you want to send to your customers and there is no monthly fee per month except VPS charges which is needed to pay to hosting service provider company like Godaddy, HostGator etc, but this will cost you only $5 per month per server and not much.
So according to me, this is the better option instead of paying 100s of dollars per month to any 3rd party company and even with you can send limited numbers for marketing emails.
So better to setup your own email server and send as many emails you want to send.
Even with is server setup you can sell emails to other like the other service provider companies. So you can use this server for your own and you share will other by getting extra money for that.