What is a Bounce in Marketing Cloud?

If for some reason a message you send is not deliverable to an email address, it can change a subscriber’s status from Active to Bounced. A soft bounce is a temporary condition, so the server will keep trying to send the email. A hard bounce is a permanent error, so the server will not retry. After multiple bounces, a user’s status may be set to Held. A status of Held will prevent Marketing Cloud from attempting future sends to that email address.

A status of Held (in addition to unsubscribes) can explain the discrepancy between the number of people on your list and the number of Total Sent. The Salesforce article on Bounce Mail Management explains all of these statuses.

You can determine if a subscriber has a status of Held in two ways. First, you can contact the Marketing Cloud admins and they can check for you. Second, you can request that a daily report, Status Change, be emailed to you. It will note if any subscriber has changed status between Normal, Unsub, or Held.

If a contact does have a status of Held, but they still want to receive your messages, you will need to contact the Marketing Cloud admins at [email protected] to have the Held status reviewed and, if appropriate, reset to Active.

https://help.salesforce.com/articleView?id=mc_es_bounce_mail_management.htm&type=5