return new Promise<void>((resolve, reject) => {
  switch (process.env.NODE_ENV) {
    case 'acceptance':
      return resolve();

    case 'production':
        transporter.sendMail(message, function (error, response) {
          if (error) {
            return reject(error);
          }
          return resolve();
      });

    default:
      previewEmail(message).catch(console.error);
      return resolve();
  }
});