When an email is returned to the sender without being accepted by the recipient’s mail server, the email is called bounce.
There 2 categories as following for bounces.
Hard Bounces – A hard bounce means the email does not exist anymore on the target server, or it has been blocked/disabled Or the target server does not exist etc.
Soft Bounce – Unlike hard bounces, soft bounces can happen for reasons like server/account temporarily unavailable, not enough disk space to store the email on the server, or
the receiving server is too busy etc.
The system will handle the hard bounced automatically and marked them as bounce email IDs in the system since those IDs will no longer valid or not in use. And for any next emails campaign, those bounced ID will be automatically excluded and will not receive any emails from you.