In email, bounce rate is the share of sends that could not be delivered, split into hard bounces (permanent, bad address) and soft bounces (temporary). A high hard-bounce rate is one of the fastest ways to damage sender reputation.
Hard vs soft
A hard bounce means the address does not exist or the domain is invalid, permanent and reputation-damaging. A soft bounce is temporary, a full mailbox or a server briefly down. Hard bounces are the ones mailbox providers read as the signature of an unverified list.
Keeping it low
Verify lists before sending, keep them tight to a real ICP, warm new domains, and pause a campaign if bounces spike. A few percent is normal; higher rates get the domain throttled or blacklisted, which is why sender reputation and deliverability hinge on it.