 Tracking Concerns Open rates is one of the earliest metrics applied in email marketing, but its continued use has become controversial due to conflicting views on its usefulness.The open rate for an email sent to multiple recipients is then most often calculated as the total number of "opened" emails, expressed as a percentage of the total number of emails sent or—more usually—delivered.The number delivered is itself measured as the number of emails sent out minus the number of bounces generated by those emails.