Emails are done via the swiftmailer. We use twig to construct the email body and I personally use smtp service to send out the email. But you are free to build your own mailing server. The one parameter you will need to configure is the app.from_email parameter. This controls where are the emails are being send from.

The service that handles all the emails is the EmailService.

Also note that by default the test environment does not send out emails.

config_test.yml