There are varieties of SMTP servers available in the plugin that you could connect to and you could create an account with many or all of them to subscribe fo their free plans, each free plans has limits of possible messages allowed within a specified period of time which may vary from one server to another.
But the cool thing about this plugin is that since it allows you to connect multiple SMTPs', it could then automatically switch from one to the other as it exhausts each of their free usage limits, thus creating that nice experience that automatically increases your sending limits.
Some of the currently available SMTP servers allowed by the plugin include:
PHP (WordPress default)
The SMTP mailers will not be able to work on your site until you’ve completed the respective processes involved in setting them up.