What is Hard bounce?
A permanent delivery failure — the address does not exist — after which the address must be removed from all sending.
A hard bounce is the receiving server saying the mailbox is not there: SMTP status 5.1.1 or similar, "user unknown", "no such recipient". It never resolves itself. The correct response is to mark the lead bounced, stop every sequence to them and keep them out of future imports. Sending to a hard-bounced address a second time is one of the strongest signals of a purchased or stale list.