I don't understand. What are use cases for using this or Mailpit? Can't you just create a new email address on existing server and check it? Why do you need different setup and different infra?
This is useful for non prod environments, especially local, where you don’t want some background job sending emails welcome to jdoe@gmail.com when you are testing the sign up flow. It allows you to safely send outbound emails to any address without worry that they will actually be delivered, without having to change your application code.