Email Open Rates HTML vs Plain Text

Open rates are only tracked in HTML emails, not in plain text emails:

Opens are tracked using an industry-standard invisible "web beacon" image embedded in each HTML email. Because it relies on a hidden image, it does not work for plain-text emails, when images are disabled in an email client or for email addresses you paste into the "additional recipients" box. As a result, the open count is generally lower than the real number of opens.