Survey Reminders are designed to be a final attempt at getting past spam filters and firewalls. By sending a plain-text email, we are taking one last shot at slipping past any spam filters that might have blocked our emails from reaching the desired recipient.
The original email we sent (with all of the fancy HTML formatting) is threaded in the same email chain, so recipients of Survey Reminders will see the original fully-formatted email below the Survey Reminder.
Survey Reminders are set on the Survey Template page - see screenshot below. They must be set individually for each Template. Survey Templates will be sent to everyone who did not open the initial email. Only one will be sent - no additional Survey Reminders can be sent for any sent Survey.