 As a result, open rates are broadly rejected as an absolute measure of a commercial email's performance.However, many marketers use open rates as a relative measure, for example to compare the performances of emails sent to similar recipient groups, but at different times or with different subject headers.The "open rate" for physical mail is difficult to quantify when compared to electronic mail but it is extremely important to the success or failure of response-oriented mail (marketing, billing, recovery, renewals, etc.