Hard Bounce
A hard bounce is a permanent delivery failure, caused by an invalid, nonexistent, or blocked email address, where the receiving server rejects the message outright and will keep rejecting it on future attempts. Hard-bounced addresses should be removed from sending lists immediately.
Why it matters for outbound teams
Continuing to send to hard-bounced addresses is one of the fastest ways to damage sending domain reputation, since mailbox providers read repeated hard bounces as a sign of poor list hygiene. Most modern sequencers suppress hard-bounced addresses automatically after the first failure. Keeping hard bounce rate low, generally under two percent, is a baseline expectation for maintaining inbox placement.
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