Short answer
HubSpot's KB: HubSpot skips contacts who permanently bounced; they show on the Not Sent tab. Use the Contact has bounced alert on the record to unbounce. Emails dropped meanwhile must be cloned and resent.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Confirm the address is fixed before unbouncing.
3. Related
Hard vs soft bounces and deleting bounces.