Bounces are email addresses that don't make it to an inbox for a number of reasons:
Hard Bounce - A hard bounce generally means that the email address is invalid; these emails will never be delivered.
Soft Bounce - A soft bounce indicates a temporary issue that's preventing the email from being received, such as a restriction on the recipient’s mailbox size.
Only Hard Bounces will be marked as "Bounced" in VerticalResponse, because we will continue to attempt to mail to Soft Bounces. We won't mail to hard bounces more than once, and those contacts will be put in the "inactive" section of your list. If you believe a person has been marked as "bounced" accidentally, then you can contact us, so that we can debounce them for you.
For more help on bounces, check out our guide Boing! Everything You Ever Wanted to Know About Why Emails Bounce.