When an email is sent from ActiveDEMAND to one of your contacts, ActiveDEMAND embeds 2 versions, an html version and a plain text version. For those recipients who have 'render only plain text' selected in their email clients, the plain text version renders. If they have 'view as html' selected in their email clients, the html version renders. These never show up as 2 separate emails, they show up as a single email, the correct version renders. When testing your emails, ActiveDEMAND can (optionally) send the two versions as separate emails to make it easier for you to review the results.