Just as opens and clicks are important to track, so are bounces, unsubscribes and blocks. These metrics tell you the number of email addresses that did not receive your emails because they either bounced or were blocked by an ISP. Unsubscribes are important to keep tab on, too, as they tell you the number of users who are opting out of receiving marketing information from you. It's important to make sure that these numbers stay small.
If you see an increase in bounced emails, try running a list cleaning service to make sure you have valid emails in your list. Always use lists where customers have opted in so that you are targeting users who are interested in hearing from you. If you see a sudden spike in blocks, look at your Spam Reports insight and make sure your emails are not getting marked as spam. Also, check the frequency of your emails; sending too many emails can result in a high rate of unsubscribes.