Check out this HubSpot article in the "Can I remove a hard bounce?" section:
In the left panel of a contact record, there will be an option to unbounce if the email address has not bounced globally.
You have the option to contact HubSpot support to ask them to remove the email address from their global bounce list.
Global bounce: a hard bounce for a permanent reason across three or more HubSpot accounts. Permanent bounces include email address is invalid and mailbox full. Global bounces are dropped from future emails across all HubSpot accounts.
Please note: when a contact's email address bounces globally, you cannot unbounce them in your HubSpot account. If the email address is legitimate, contact HubSpot support to unbounce them. However, certain bounce types such as unknown addresses and full mailboxes cannot be unbounced because these inboxes are likely abandoned.