For now, we will just create the MailManager interface and an empty implementation, DefaultMailManager. We will finish the implementation later in the next chapter. The following is how the MailManager interface looks:

public interface MailManager {
  void send(String emailAddress, String subject, String template, MessageVariable... variables);
}

As you can see, the API is quite simple. It requires the recipient's email address, the mail subject, the template of the mail message, and the variables that are used inside that template.