The percentage of people who’s email address was not deliverable. It is important to track email messages that are not delivered and prune them from your list to stay in good graces with email service providers. To calculate your bounce rate, you take the number of emails delivered and divide it by the number of emails sent. For instance if we sent 1,000 emails and 20 of them came back as ‘undeliverable’ the bounce rate would be 2%. To calculate this we take 20 / 1,000 = .02 and multiple .02 * 100 to get the whole number percentage of 2%.