The header "Content-Type", if available, will be respected if you enable this setting.
        Settings such as e-mail format ("text/plain" or "text/html") and character set may be provided through this
        header. Unless your site somehow alters e-mails, enabling this setting will result in all e-mails to be sent
        as plain text as this is the content type Drupal by default will apply to all e-mails.