In order for your applications to be able to send out emails, you need an SMTP server. A nice free one is Mailgun.
Create an account, and follow their steps to register your domain.
Once that is completed, you can plug the settings they provide into
settings/config.yml file. To have the
mailserver stack use
this SMTP server to send rather than trying to send mail out directly
mailserver: forward_to_smtp: True