Hard bounce
A hard bounce is a permanent delivery failure, such as an address that does not exist, and you should never email that address again.
A hard bounce comes back with an SMTP error in the 5xx range. The most common is '550 5.1.1 user unknown', which means the mailbox does not exist. Other causes are a domain that does not exist, a typo in the address, or a person who left the company and had the account deleted.
Retrying a hard bounce never helps and it hurts you. Each attempt to a dead address tells the provider that you do not clean your list. Remove the address from the campaign and add it to your suppression list so it is never mailed again.
Be careful with codes that start with '5.7'. They are policy blocks (for example, a blocklist or failed authentication), not a missing mailbox. They say something is wrong with you, not with the address. If many hard bounces share a 5.7 code, check your DNS and your blocklist status instead of deleting the contacts. To reduce hard bounces, verify lists before sending.
In Terlido
Terlido counts hard bounces for each campaign and uses them to decide whether to auto-pause the campaign.