An open rate is expressed as a percentage of the total number of recipients who viewed or opened your e-mail campaigns.
Open tracking is measured using an < img > tag embedded in the HTML code of your outgoing emails. This code causes a small, transparent tracking image (not visible to the recipient) to launch when an email is opened. When the image is launched, an "open" is recorded for that recipient. SimplyCast compiles real-time, easy-to-interpret reports showing the percentage of opens and who opened your email.
Because open tracking is done using a small image in each send, you cannot accurately track plain text emails.
Some recipients may open the e-mail campaigns but will not enable images, so this metric will be somewhat inaccurate. Nevertheless, open rate remains a useful gauge of email campaign performance.