You can set up all of your notifications to be sent in HTML format or as plain text (email notification, autoresponder, copy of the submission, payment notification, approval email, quiz template). To send notifications in HTML format or as plain text, simply create a template. After editing it, click the Body’s dropdown list and click on Switch to Plain Text.
Click Save at the bottom of the page and you are all set.
If you are using another version, check out this article with the same subject.