Bounced vs. Undelivered Messages
Your report separates messages that did not arrive at the destination into two categories: Undelivered and Bounced. The difference between these two types of failures is whether or not we believe the error is correctable or not.
Undelivered messages are soft bounces. Soft bounces could eventually be resolved. Issues such as full mailboxes are examples of what should show up when you export your undelivered addresses.
Bounced messages are hard bounces. Hard bounces cannot be resolved easily. A specific domain not existing is an example of a hard bounce. For example, if you emailed firstname.lastname@example.org when the email address was email@example.com, this would show up as a bounced message. Addresses that result in a hard bounce are set to inactive. We advise not re-activating hard bounces because these email addresses are not good and will likely never be delivered to.