How can I send encrypted mail?

I’m able to send mail using drupal_mail with the following code

drupal_mail('custom_module', 'custom_key', $email, language_default(), $params,'[email protected]');

My hook mail is written like this

function custom_module_mail($key, &$message, $params) {
  $feedback = $params['result_id']; // feedback contains HTML elements
  switch ($key) {
    case 'custom_key':
      $message['subject'] = 'Your feedback on the survey ';
      $message['body'][] = $feedback;

However when I try to open the message in my inbox it is not showing email body, but it is showing the email is unencrypted with the icon. Also when I look at the mail details, it points to learn more with following…

How can I ensure that mail I have sent is encrypted?

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