You can see what emails have bounced back when you click on Marketing and then tracking. In your list of campaigns, there is a column labeled bounced that will show how many bounced emails have occurred from a specific campaign.
To see what email addresses bounced, hover over tracking and click on bounced. This will bring you to a list of instances when an email bounced. You can click on one of the email addresses to bring you to a contact entry to update it if necessary.