By default TurnKey Linux appliances send email directly. However to provide more robust email delivery, it is highly recommended to use a third party SMTP relay.
We have researched the available third party options and have concluded that SendinBlue are currently providing the best mix of features and value for money.
A free SendinBlue account should provide sufficient capabilities for general low-medium email traffic use. If you wish to send mass-marketing emails or are sending large volumes of transactional emails, you may be better served by upgrading to a paid plan.